Researchers have found that if toddlers engaged in social sports or

Several issues on display here. First let's look at the end of the sentence where you have a nice split between "than" and "than do". This is a comparison issue as you are comparing the "sporty/social" toddlers with the "anti-social" toddlers. As such, including the word "do" incorrectly compares the "social" toddlers with an action. Eliminates A/C/D.

You also see an issue around the verb tense related to the "if clause". You need a tense pairing that makes sense for an "if clause" - past/past, present/present or present/future.
A - past/future (wrong)
B - past/present (wrong)
C - present/future (fine)
D - present/present perfect (wrong)
E - present/present (fine)

There is also a nice adverb/adjective split here - more quickly/quicker. Here we need the adverb "more quickly" to modify the verb "develop", not the adjective "quicker", which we would use to modify a noun. Eliminates B/C.

That leaves answer choice E.
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The sentence relates to the IF - Clause and Modifier. In IF clause rule, choice C and E are fine. However, in the meaning, choice C using QUICKER that is adjective and modifies wrongly the clause before it, while choice E using MORE QUICKLY that is adverb and correctly modifies the developing... and learning...

So, choice E is the final answer choice.
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Hi KyleWiddison or any1 else,can you please explain the If clause (whats should one keep in mind when if is used)

The 'if' clause isn't commonly tested on the GMAT, so you shouldn't worry about it too much. In short, the 'if' clause presents an 'if' condition with a 'then' outcome. Example: If I go to sleep late, I will be tired tomorrow. There are lots of ways to create the if clause correctly (present/present, present/future, past perfect/conditional perfect...). Here is an example with past perfect/conditional perfect: If I had gone to sleep late, I would have been tired in the morning.

One thing to note, the GMAT won't use helping verbs "would" or "should" in the 'if' portion of the clause. WRONG: If I WOULD go to sleep late, I will be tired.

Hope that helps!
KW
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A very good practice question.
"If " brings a subtle cause-effect in the sentence,so (C),(E) seems to be the right choice.
"More quickly" is the right choice over "Quicker than" .

(E) wins

I am confused between choice C and E. I guess because of the usage 'than do' and 'than'n the end I will go for E

Do you think that (do) at the end is necessary to complete the comparison. because we compare the development between who engage in social sports and who do not do?