Hi Akhilesh--

Well, EssaySnark is no expert, but here's what we can offer:

This sentence would've still been correct if "and" were included. As we can see from the OA, it is not incorrect without it! Yes, confusing!!

The only way that we were able to solve this one was through pure elimination. We definitely did not recognize that E was correct upon first reading it. Instead, we were able to eliminate all but C and E, based on other errors. Then, we eliminated C because "so" was missing. Which leaves us with E as the only choice.

The usage in this sentence is uncommon but that's what the GMAT is testing for, isn't it??

You probably already broke it down this way, but we see this problem as needing to be analyzed twice:

Does the first phrase work with the second part of the sentence?

"So dogged were Frances Perkins' investigations of the garment industry that Alfred E. Smith and Franklin D. Roosevelt recruited Perkins to work within the government, rather than as a social worker."

Does the second phrase also work standalone?

"[S]o persistent [was Frances Perkins'] lobbying for wage and hour reform that Alfred E. Smith and Franklin D. Roosevelt recruited Perkins to work within the government, rather than as a social worker."

This is surely not an official grammar rule, but EssaySnark sees it as the commas setting off the "lobbying" clause to be serving as an implicit "and".

We know our explanations are wobbly... hope this adds some value nonetheless!!

EssaySnark