Explanation

Creating a filter: supposing that we can't generate a filter easily, we can move on to the answer choices (B) through (E) with the aim of finding objective defects and eliminating one by one.

Finding objective defectives: choice (B) is easily out, because it uses the singular verb "is" to refer to babies. Choice (C) is out; the word "that" is meant to introduce an indirect statement, but what follows is a run-on. (E) is clearly inferior to (A), since "have recognition of" doesn't clearly express ability, which is the point of the intended sentence. That leaves us with (A) and (D). Are the babies estimated "to be" 9 months old or estimated "at" 9 months old? We are down to an idiom, and the correct idiom here is "to be," so the answer is (D). The word "estimated" is followed by the phrase "to be" to indicate that one thing has been estimated equal to another. The phrase "to be" will not be correct everywhere. For example, if the verb were "considered," we would not need any words in the middle at all, we could say "the babies were considered 9 months old" (though the meaning of that sentence is odd), not "to be 9 months old." The correct answer is (D).