Given that D is undesired reaction,how do you size the reactor in terms of the rate of consumption of B.Am i allowed to use the rate of disappearance of A which is in excess?
It looks like you're using the wrong reagents to produce C. What you should do is find a different collection of reagents that will produce C. Now if C freely dissociates like an ionic salt in water (A+B↔C) then you're out of luck as long as you try to do this in an aqueous media.
(I love telling a student that the teacher has set a trap. Good luck using my answer. )
__________________
"Don't disturb my circles." translation of Archimedes last words