The processes of nitrification and nitrate assimilation are

The correct option is A oxidation and reduction respectively
Oxidation often involves the gain of oxygen. Nitrification is the process by which ammonia is converted to nitrites $\left(N{O}_2^{-}\right)$ and then nitrates $\left(N{O}_3^{-}\right)$.

Reduction often means the loss of oxygen. Nitrate assimilation is the biological processes in which nitrate is converted to ammonia, which can then be incorporated into organic compounds.