Dr. Sayre’s lecture recounted several little-known episodes in the relations between nations that illustrates what is wrong with alliances and treaties that do not have popular support.

(A) relations between nations that illustrates
(B) relation of one nation with another that illustrates
(C) relations between nations that illustrate
(D) relation of one nation with another and illustrate
(E) relations of nations that illustrates

B or C? WHY?

The episodes illustrate, not the relations. This is one of those rare cases where the noun does not sit right next to the modifier. You're allowed to have a short prepositional phrase in the middle IF that phrase is necessary to understand the meaning of the "main" noun (in this case, episodes). If we just use the word episodes without description, we don't really know what we're talking about - we need to describe episodes with "IN the relations between nations."

So, in B, "illustrates" doesn't match. "of one nation with another" is clunkier than "between" but notice that they don't make us decide just on "that sounds awkward." Even in D, which does correctly use "illustrate" - it changes the word "that" to "and," and that changes the original meaning of the sentence (the second part would now be some thing separate, like another thing he recounted during the lecture - and then it wouldn't be parallel).

hi Stacey, But the OA is B

I too came up with C , just as you did.
Can someone help here?

there is absolutely, positively no way that the official answer to this question is (b).
(c) is the best answer by a country mile.

two things:
1) while this is still a good practice problem - it tests SV agreement, idiom, and clarity - it definitely sets off my "non-official radar". it's rare not only for real gmat problems to contain long, unbroken, difficult-to-follow sentences with NO internal punctuation, but also for them to contain relatively wordy / informal constructions (e.g. "what is wrong with" vs. the much more formal and concise "(the) problems with").

2) if this problem is taken from a source that indicates (b) as the correct answer, run, don't walk, away from that source. run for your life.
if you can set fire to it while simultaneously running for your life, all the better.
there is absolutely no way that this is an official problem with official answer (b), unless there's been a switch-up or typographical error (which does occasionally happen).

thanks Ron and Stacey ..I indeed saw this question at some other source, which mentioned the OA as B. :D
OA should definitely be C
:-)

Right - trust Ron and Stacey. And PLEASE, only post questions in the "GMAT Prep Verbal" section if you get them from the GMAT-prep software. Thanks.