By Friday evening the proposal had been signed by

By Friday evening, the proposal had been signed by over 1000 supporters, many of them has left messages on the page.

(A) had been signed by over 1000 supporters, many of them has

(B) was signed by over 1000 supporters, many of them having had

(C) had been signed by over 1000 supporters, many of whom have

(D) had been signed by over 1000 supporters, many of whom

(E) was signed by over 1000 supporters, and many of them

POE (Process of Elimination)

• Split #1 - we need PAST PERFECT for the description of one kind of event

The phrase By the time X, Y indicates a time shift. At least one verb must shift to reflect that time shift.
The Y event happens before the X event.
Write X in simple past tense.
Write Y in past perfect (had verbED).

So by Friday evening signals that an event that occurred before that time should be depicted in past perfect.

Think about meaning and emphasis. The sentence is saying, "Wow, BY (at the time of, when that time hit) Friday evening, a lot of people HAD signed the petition!"

"By Friday evening" was in the past.
-- X event = simple past tense OR time marker
X = By Friday night
Signing the petition began before that time and was measured up to that time.
-- Y event = past perfect = had been signed

Options B and E incorrectly use simple past tense: the proposal WAS signed by 1,000+ supporters

Eliminate B and E

• Split #2 - Subject verb disagreement
Option A incorrectly pairs many of them (plural) with the singular verb has
Eliminate A

• Split #3 - Verb tense
Option C incorrectly uses . . . many of whom have left messages on the page.
The present perfect tense in HAVE LEFT is inappropriate.
-- the messages do not continue into the present
-- the effect of the messages does not continue into the present ("effect continuing into the present" does not mean that merely because the messages exist, they exert effect)
-- the action was not performed during a period that has not yet finished
-- the action is not repetitive and during an unspecified period of time between the past and now
-- the action was not completed in the very recent past and is not accompanied by the word "just."

We have two choices
(C) many of whom HAVE left messages
(D) many of whom LEFT messages

Which is better?

We are not given a past perfect option, many of whom had left messages. in D, the author chose a somewhat idiomatic construction in which whatever the subgroup indidentally DID is in simple past.

In English, we can "tack on" an another event in simple past tense. We have conveyed the correct emphasis using had signed. If we have never seen an answer like D, we choose it because C makes no sense.

The people who left messages did not do so after Friday night. If, as C suggests, the messages were still having effect in the present (?), then the acts of signing certainly would also. In that case both options would state, "By Friday evening, more than 1000 supporters HAVE signed the petition."

BY THIS time is a cutoff that marks off a time in the past. Whatever comes before is also in the past. (C) is illogical.

The sentence is describing something incidental that was the case BY Friday evening.
Many . . . LEFT messages.
LEFT messages was the case on Friday evening = simple past

Similar, correct: By the end of the marathon, more than 100 people had developed heat stroke, some of whom required IV fluids.

When in doubt, choose the option that is not nonsensical.

The correct answer is D

OTHER ISSUES and ANALYSIS

• Another issue: Comma splices.
Two independent clauses (a clause with a subject and verb that can stand on its own) may not be joined by a comma alone.
Wrong: The woman made a joke, the man laughed.
Correct: The woman made a joke, AND the man laughed.
Wrong: The woman made a joke while dining with friends, all of them laughed.
Correct: The woman made a joke while dining with friends, and all of them laughed.
Correct: The woman made a joke while dining with friends, all of whom laughed.


Many of tells us more about a certain subgroup of the supporters.
What did many of the supporters do? They left messages.
When we add this information, though, we cannot simply use a comma + many of them.

Many of them is a standalone subject with its own verb: Many of them left messages.
-- That sentence is an independent clause: subject, verb, and can stand on its own.
-- The first part is also an independent clause:
By Friday evening, the proposal had been signed by over 1000 supporters.

We cannot "stick" or "glue" those two italicized independent clauses together with nothing more than a comma.
We need a comma and a conjunction.
Sticking two ICs together with just a comma is called a comma splice and is ungrammatical.

Options A, B, and E create comma splices by using many of them.

• non-essential relative clauses

A relative pronoun such as who often follows a comma and modifies a previous noun but does not create another independent clause:
She was captivated by a relatively new musical artist, who was both gifted and underrated.
WHO refers to the artist and tells us more about him.

Non-essential relative clauses (e.g. who-clauses set off by commas) can be introduced by expressions such as some of or many of followed by the relative pronoun.
When we use prepositions such as OF, we use the relative pronoun in the objective case (i.e., it can be an object of a preposition: me, him, her, us, them, whom)

She was captivated by a quite a few relatively new musical artists, many of whom were both gifted and underrated.
Whom refers to the artists.

Relative clauses are not independent clauses, so they do not create comma splice problems in this case.

Options C and D correctly use a non-essential relative clause (though C gets the verb wrong).

-- Many of whom left suggestions.
-- NOT a complete sentence. (Good. We do not want a complete sentence. Then we would have a comma splice.)
-- Whom is an OBJECT pronoun.
The object of a preposition can NEVER be the subject of a clause and thus cannot create a independent clause.

• Verb tenses: REDUX

By the time X, Y is a signal that time shifts must be reflected by verb tense shifts.
Y event or events happened before the X event.
X takes simple past tense. It is later in time.
Y takes past perfect. Past perfect is called "the past of the past."

Usage of past perfect?
Well, in order to signal the past of the past, first we need a marked-off moment in the past.

Past perfect
(1) requires at least one event rendered in simple past or a time marker
BY FRIDAY EVENING is the time marker of that past.

(2) usually is NOT coupled with sequence words such as before, after, previously, subsequently.

To learn more about By the time X, Y
please see my posts on THIS THREAD, HERE.

ONE POST is HERE
Another post contains more explanation and a diagram. It can be found HERE

The answer is D

COMMENTS

liaoliao , Annet1524 and Xima , welcome. (Xima , I remember your username, just can't recall whether I've seen you on Butler)

This question is challenging.
It is also fair.
By Friday evening, how many had signed?
More than 1,000.
So, there were more than 1,000 supporters who, while signing, LEFT messages.

We can have a past perfect and more than one simple past tense event.
By the time means "at this point in time."
Please see the links I gave above.

Arvind42 , LUCIFER1703 , Ashokshiva , Doer01 , and baru all have correct answers.

Nice work.

The rest of you display pretty good reasoning.
As is characteristic, GKomoku left us with the best graphics on GMAT Club, plus a thank you. Definitely kudos.
Incomplete explanations receive a smiley face. Highlighting an error without explanation is . . . not explanation.

I am happy to see you all here.

Kudos to all.