For which one of the following reason he went down in language skill.A second grader (ESL) missed months of classes for chronic illness. As a...

The answer is all of the above, but the most important one
is 2: Lack of peer interaction. There are indeed optimal moments for learning in the
development of individuals, but when it comes to language, the best way to extend it is
by putting it into practice whether it is within the content area or
not. 

Incidental, interactive, and communicative language
are proven to be THE most effective ways to acquire an L2 much more so than formal
grammar and language lessons.

This student probably stayed
home being tended to by family members who speak the L1 and he cannot communicate in the
L2 with his peers. Whatever lesson he missed has nothing to do with his expanding
language, and opportunities for learning appear at all times throughout one's
lifetime.

Therefore, the answer is
2.