20) C

Oxygen is only required for active transport. Active transport is the movement of molecules from a region of lower concentration to a region of higher concentration. (opposite of diffusion)

A – Wrong
Cytosine will move from L to K from a region of higher concentration to lower concentration. So movement of cytosine from L to K would not require active transport and hence no oxygen.

B – Wrong
Glucose will move from K to L from a region of higher concentration to lower concentration. So movement of glucose from K to L would not require active transport and hence no oxygen.

C – Correct
Magnesium ions will move from L to K from a region of lower concentration to higher concentration. So movement of magnesium ions from L to K would require active transport and hence would require  oxygen.

D – Wrong
Nitrate ions will move from K to L from a region of higher concentration to lower concentration. So movement of nitrate ions from K to L would not require active transport and hence no oxygen.