OFFICIAL EXPLANATIONThis question is probably flawed.
The writer of the question pegged the answer as D.
I think that A is as good or better.
Very rarely, as is the case in option A, we are allowed to use "higher" without "than" when the object of comparison is obvious.
I am trying to find the official question in which I recall seeing this construction.
On the other hand, option D does not use a comparison word such as "higher" that almost always requires a "than."
We have a comparison in the form of contrast.
In option D, the dependent clause begins with
while and leads with adipose tissue (fat).
The main clause leads with small mammals.
The meaning seems pretty clear: the bodies of small mammals and newborn babies have a high proportion of brown to white fat in their bodies, whereas large mammals have mostly white fat.
Meaning trumps everything, including parallelism.
Just one problem. In option D, I cannot tell whether "higher proportion" means brown fat:total body or brown fat:white fat.
Both A and D are defensible.
Neither is great.
The question is archived.
Everyone gets kudos.