(B) says, "innate mechanisms are a contributing factor in language learning"
The problem with (B) is that the argument is already giving us this information in the stimulus, so it wouldn't make sense for it to be completing the passage when it's already been mentioned. Thus, (B) is out, as it's redundant. This may have been why you found (B) tricky (because it's mentioned in the stimulus already).
(E) says, "innate mechanisms play a more important role in language acquisition than a child's immediate environment"
(E) is great because it discusses the big question that we're looking to know. Based on the previous information from the stimulus, we want to know whether environment or innate mechanisms are more important, so (E) is the correct answer choice.
Hope this helps. Let us know if you have any questions!