(A) "developed only after when" is needlessly wordy
(B) "developed only after when" is needlessly wordy
(C) "developed only after Harry Truman's support of civil rights legislation": correct!
(D) "develops only at the time": incorrectly changes the verb tense
(E) "developed only after there being" needlessly wordy and awkward

answer: C

Nice explanation, TheGMATTutor I've always liked this question as an example of the GMAT's preference for brevity and simplicity.

The word "after" connotes that we're talking about time, so the word "when" is redundant and unnecessary.

As a few general rules that you should find helpful:

1) If you can't find a grammatical difference between two sentences but one is shorter/more concise, the shorter one is probably right. Business writing is about being direct and to the point...there's probably a grammatical reason that the shorter one is better (redundant terms, etc.) but for a quick decision you can feel pretty confident in the shorter option if you haven't been able to rule out either on other terms.

2) When sentences have connectors between multiple clauses (like comparisons or lists), check the connectors to see if they use unnecessary or incorrect words. It's kind of a "needle in a haystack" thing that they test - the clauses being connected can be made long and detailed to distract you from the problems with the small connectors, so know that you'll likely want to check those connectors, and that can help you avoid having to sift through the longer phrases, too.