To meet the rapidly rising market demand for fish and seafood, suppliers are growing fish twice as fast as their natural growth rate, cutting their feed allotment by nearly half and raising them on special diets

A) their natural growth rate, cutting their feed allotment
B) their natural growth rate, their feed allotment cut
C) growing them naturally, cutting their feed allotment
D) they grow naturally, cutting their feed allotment
E) they grow naturally, with their feed allotment cut

I initially selected C, but that is incorrect according to GmatPrep. OA is D. Can someone explain the difference between C and D and why is D is correct. Also what is "their, they, them" referring to? What is plural - fish?

Thanks!

Cutting in d completes the parallel in that ends in ing...also plural of fish is fish.

But cutting in C is placed in the same place. Can someone clarify between C and D? Thanks!

C) growing them naturally, cutting their feed allotment

growing them naturally doesn't make much logical sense. You 'let them grow naturally' , i.e., without intervention.

D) they grow naturally, cutting their feed allotment

The fact that there is no human intervention is clear here.

agreed on all counts.
well done.