One of four babies are now born to mothers aged thirty years or more, compared with just one of six born in 1975.
(A) of four babies are now born to mothers aged thirty years or more, compared with just one of six born
(B) of four babies is now born to a mother whose age is thirty or older, compared to just one of six babies who were born
(C) baby in four are now born to mothers aged thirty or older, compared to just one in six
(D) baby in four is now born to a mother aged thirty or older, compared with just one in six
(E) baby in four is now born to mothers aged thirty years or more, compared to just one in six

OA is D. Why is E wrong please

Classic Compared with vs Compared to example.

Two things that are similar are compared, hence usage of compared with

Not sure as Why A is ruled out ...Maybe Ron/Stacey can give more insights

I would choose D because the reference is to one baby and not four. So the singular 'born to a mother' should be used rather than 'born to mothers' (as in E).

Notice the split in 'is/are'. Since we are talking about one baby, 'is' is right, hence ruling out A.

the subject is 'one baby'.
one baby has one mother.

choice (e) is wrong because it implies that one baby can have 'mothers' (plural).

nb: the fact that 'one in baby in four' is a statistic that actually represents a large number of babies (25% of all of them) is irrelevant; grammatically, we're still talking about literally one baby.

Ron - I have a question here.

If choice (E) were to be rephrased as "baby in four is now born to a mother aged thirty years or more, compared to just one in six", what would be the correct answer choice? I feel that it still should be (D), because 'compared to' doesn't fit in here where the sentence refers to differences instead of similarities. Please assist in helping me understand the corrsect usage.

Thanks.

that would be a tough call; it would turn entirely on idiomatic usage (namely, "compared with" vs. "compared to"). i've seen both of those expressions in common usage, and the gmat may well consider both of them correct; i have yet to see any hard-line evidence favoring either one exclusively over the other.
also, although i don't have a good reason why, i'm quite sure that "aged 30 years or more" would be considered awkward usage, especially vis-à-vis the more conventionally acceptable form "aged thirty years or older". you can call this idiomatic usage if you want; i don't have any better reason for it other than that i've seen the preference in common usage.