OA is D
Spot the Concept Being Tested:
Pronouns: Look for pronouns in the underlined part.
In this case the pronoun they・is used to refer to children.
Lists: Look for multiple similar ideas, for conjunctions, or for multiple descriptions.
In this case there are two lists.
The first is associate ... and play ...The second is experience ... and develop ...
Comparisons: Look for comparative triggers.
In this case quicker ... than indicates a comparison between the children.
Decide on a Fix:
The rule for Pronouns is that the pronoun must clearly and correctly refer to one thing.
In this case the pronoun they could technically also refer to studies, but where there is ambiguity between a person and a thing when using they on the GMAT the test assumes it refers to the person.
So the pronoun is fine.
The rule for Comparisons is that the comparison must be of comparable items and must be balanced.
In this case the comparison quicker than could only apply to the children.
It would refer to their speed.
This is not the intent of the sentence.
The correct comparison should compare the development of the children.
Therefore the correct comparison should be more quickly than.
The rule for Lists is that the list must be of similar items and balanced.
The first list is of two verbs in the same time frame that are similarly constructed.
It is fine.
The second list is also of two verbs in logically consistent time frames and of similar construction.
It is also fine.
POE:
Use the Comparison Error to eliminate choices.
Only Choice D and Choice E make the valid comparison of verb to verb.
Eliminate A, B and C.
Chunk and Compare:
Compare D to E.
The first difference is play in Choice D versus played in Choice E.
Choice E changes the first list to make in unbalanced.
Eliminate E.
Reread Sentence With Your Choice and Choose an Answer.
Choose D.