I thought the correct answer would have to be E. To answer your question, I don't think the helping verb "do" is necessary when the comparison isn't ambiguous. Here in this question, the toddlers who attend certain events are "developing" skills is compared to other toddlers who do not attend these events; there's no confusion in which the toddlers could develop other "toddlers".

As for the reason why I choose E:

1. The non-underlined portion "those who do not attend such events." is in present tense, hence the main verb of the comparison should parallel and both be in present tense.
2. Quicker is not the proper adverb, more quickly is.
3. "have developed" present perfect is the wrong tense, and the second verb "learn" should be in past participle as part of parallelism "have developed" and "have learned".
4. "engaged" should be followed by "would", not "will".

However, I did noticed that E is missing a modal verb.

also pick up E, and it totaly agree with above said
but small note
Quicker is really not an adverb at all, it is comparative adjective, with superlative form-quickest
so quicker can not be used to modify verb learn

Thanks guys for your contributions to my question above.

e-GMAT wrote:This question appears to be very similar to the question discussed in the thread below.

https://www.beatthegmat.com/botanists-ha ... 66153.html

I suggest you go through the discussion in the above link and apply the fundamentals of if..then constructions in the sentence in this thread. Let me know if you have any questions.

Also, I am curious about the source of these two questions (1 in this post and the other in the thread indicated above)

Regards,

Payal

Great explanation in the other thread.