IMO D.
Reason:
A:their natural growth rate, cutting their feed allotment :referrent for their is not clear.It is not clear whether their refers to the fish or the suppliers.
B:their natural growth rate, their feed allotment cut:Violation of parallelism,growing and their feed allotment cut not parallel.
C:growing them naturally, cutting their feed allotment :we are comparing how fast suppliers are growing fish (unnaturally) and how fast the fish grow when left alone in nature (naturally). This choice incorrectly implies that suppliers can grow fish naturally.
D:they grow naturally, cutting their feed allotment :CORRECT
E:they grow naturally, with their feed allotment cut:Violation of parallelism as in the case of option B above.
Cheers!!
Amit