Since this is NOT a typical comparative adjective that requires a mandatory "than", it would probably be better to memorize it as an idiom.

This usage may be considerd as stating an action and then providing an additional reason for taking the action - a reason that made someone take the action with stronger justification.

I went to the party, more so because she was there.

Fact: I went to the party
Additional reason that provided stronger justification for my action: because she was there.

Removign "more so" would not convey the sense of this additional justification for my action.

Moreover the preceding post from sukanyar is helpful in understanding the meaning better.

Hi,
the initial phrase -Like the Dreyfus affair at the turn of the century and the Sacco-Vanzetti trial in 1921- correctly modifies 'teh hiss'.. also it is part of non-underlined portion..
So our Q mainly is - the Hiss case became the political cause of an era, the more so because it was the first time a congressional hearing was shown on television.

lets see each statement


Like the Dreyfus affair at the turn of the century and the Sacco-Vanzetti trial in 1921, the Hiss case became the political cause of an era, the more so because it was the first time a congressional hearing was shown on television.

1. the more so because it was..
here 'more so', a kind of an IDIOM, is correctly used to refer to entire preceding clause...
MORE requires 'so' here to make sense

2. more for its being
requirement of So...
its being is not a better construction then A

3. more for the reason that it was
Same SO is missing..
SO is must to connect to previous clause

4. the more so for the reason because it was
more so is correctly used, but BOTH reason and because are redundant

5. the reason of it being more so is that it was
Clearly awkward construction

A

This is what http://www.merriam-webster.com/dictionary/(all)%20the%20more%20so says about "the more so":

Used to say why something (such as an attribute or quality) applies to a greater degree or extent <The play was impressive—(all) the more so because the students had written it themselves.

As a non-native speaker, I was also totally clueless on this one.

I think C is incorrect because of lacking of "that" after "more for the reason". B is awkward, D and E is redundant. So A is the correct one

In the correct choice (A), "so" refers to an entire action introduced in the previous clause: "became the political cause". This usage implies: "the Hiss more became the political cause."

Not using "so" does not re-introduce the previous clause. Therefore A is better than C.

Hi sayantanc2k has very aptly talked of importance of 'so' or any other word to relate MORE to previous clause..
In C, there is no clause/ phrase/adjective to which MORE is refering to..

Could you elaborate on the usage of "the more" here?
Since I am not a native speaker, I am not sure why we can put "the more" after the comma.
Is this an idiom or a common usage in standard English?

mikemcgarry or daagh Could you please provide more explanation on how to eliminate C in this question?

A. the more so because
B. more for its being 'being' is generally wrong on GMAT. additionaly, 'more for its being it was...' is very awkward
C. more for the reason 'for' and 'reason' are redundant. 'For' here is representing 'because', which has same meaning as 'reson'
D. the more so for the reason because. 'reason' and 'beacuse' are redundant
E. the reason of it being more so is that very awkward structure

A is the answer
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thangvietnam, it's just an idiom. "The more so" basically means "even more so." Here, it could be seen as saying "and this was even more true in this case."
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Experts,

Please help, still not clear why C is wrong.

Please refer to this post:
like-the-dreyfus-affair-at-the-century-and-the-sacco-vanzetti-trial-in-216997.html#p1675309

"So" is required to refer to the event "became the political issue". Option C could still be considered if it were "...more so for the reason that...", but even then option A would be better because of the more concise "because" instead of " for the reason that". "More so" can be considered an idiom and in absence of "so", the comparative adjective "more" would require "than" - the structure then would be " more for the resaon X than for the reason Y" OR "more because X than because Y".

GMATNinja

Sir I marked the answer as C. I read all the comments above but none could explain in depth as to why option C is wrong?

Please help.
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(A) the more so because it was- Correct
(B) more for its being--Incorrect usage of being
(C) more for the reason that it was-Wordy as compared to A
(D) the more so for the reason because it was- reason , because together wrong
(E) the reason of it being more so is that it was- Wordy

I can try to help. I took two simple sentences to understand which structure out of the two choices (A and C) is better.
Steve gorged on every food he could find. He did so because he was very hungry. (We need so to represent the action - became.)
Now compare the above to this - Steve gorged on every food he could find for the reason that he was very hungry. ( this one is too wordy.)
It is more concise to use because instead of for the reason that.

In (A), "because" starts a subordinate clause and in that clause "it" is parallel with the noun of the superordinate (main) clause, "the Hiss case".
In (C), "that" starts a relative clause that only modifies "the reason" rather than the main clause; thus, in this option, "it" is a bit more ambiguous.

This is another reason why (A) is better than (C), but probably not the most important reason.

generis Please help.

Why is option C wrong??
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Ok so, clearly every one who has a doubt is between C & A.

I will elaborate it in detail but donot skip what I am saying coz you need complete approach.

AS B,D,E are wrong I won't waste time on them.

LETS ANALYSE C,
Try to read the entire clause after comma,

More for the reason it was the first time a congressional hearing was shown on television.

Do you see the awkward construction?
IF YES, congrats.

Want more?
HERE
we need a "that" after reason, everyone was correct to say that so relates to the previous clause,

Try to see it this way
X more so because Y
X more for the reason Y

X & Y must be independent clauses

sub ordinate in case of because. Search for types of clauses and see their usage. Download SC grail by aristotle. Do some research on usage.
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Excellent explanation chetan2u