SpaceX was founded in June 2002 by Elon Musk, whose goal was to build

OFFICIAL EXPLANATION


THE PROMPT

• Notice that Elon Musk had a goal. A goal strives to create something that does not yet exist.
• The main issues tested in this sentence are
(1) which vs. that
(2) verb tense: how to convey a hypothetical

THE OPTIONS


• which is not correct
-- which should be preceded by a comma because it introduces nonessential information
In (A), no comma exists before which.
The fact that the rocket is supposed to return is central to the meaning of the sentence
→ which and that are not interchangeable on the GMAT
-- which: introduces nonessential information and is preceded or set off by commas
-- that: introduces essential information and is almost never preceded by a comma
→ in British English, which and that are interchangeable.
If you speak B.E., be a bit careful.
→ Yes, GMAC still tests the issue.
SPOILER ALERT In OG 2020, SC #824 explicitly tests the which/that distinction.
That official question is here.
• returns is not the correct verb tense. See option B.
ELIMINATE A

• returns is not the correct verb tense
→ the sentence describes a hypothetical event, a fact that we can figure out from the word goal
→ simple present cannot be used to convey a hypothetical event
• if you are not sure, keep B (and then discard when you reach option D)
ELIMINATE B

• which creates the same problems as those in (A)
• will return is simple future tense, which is not used to describe a hypothetical event
ELIMINATE C

• I see no errors
• the condition could return works well because it describes a hypothetical event
KEEP D

• this option creates an illogical fragment
→ having returned is used to show that an event has already occurred, but this sentence refers to an event that has not yet occurred.
ELIMINATE E

The answer is D.

Suppose that you kept B and D, but you're stuck.
Re-read the sentence, looking for clues, perhaps subtle, that the sentence is describing a hypothetical.
Look for a word that connotes the future or the unreal past.

You should notice that the word goal indicates an event that has not yet happened.
Bingo. Could return is better than returns. Option D is the answer.

COMMENTS

vanshi2301 and NitishJain , welcome to SC Butler.

Most of these answers range from very good to outstanding.
tyildirim92 wrote an interesting tidbit:
on behalf of the GmatClub SC Butler Project family, I would like to send my best regards to the astronauts, Robert Behnken and Douglas Hurley, who embarked on the Falcon 9 project recently

Very cool. Indeed we will send best regards to those astronauts. Thank you for including this interesting detail.
What if they Google themselves and find your shout out? Kinda awesome.

These answers range from good to excellent. Nice work.