(A) tells us why the students who learned to write letters most automatically were able to do so. But this doesn’t help connect writing letters automatically to improved composition skills.

We already know there’s a correlation in the study between the most improved composition skills and learning to write letters the most automatically. (B) doesn’t reveal any new information that suggests a causal connection.

This strengthens by showing that the correlation observed among the ones who had learned to write letters the most automatically was also observed among the broader group.

Representativeness wasn’t an issue because the author’ didn’t assert that every first-grader could achieve the same results observed in the experiment. The conclusion was simply that there’s a causal relationship between writing letters more automatically and freeing mental resources.

We already know there’s a correlation in the study between the most improved composition skills and learning to write letters the most automatically. (E) doesn’t reveal any new information that suggests a causal connection.