Processes of nitrification and nitrate assimilation are

The correct option is A oxidation & reduction respectively
During nitrification, nitrogen in ammonia loses electron, so it is oxidation.

During nitrate assimilation, nitrogen gains electron to form inorganic compounds. So it is reduction.

Nitrification is the process that transforms ammonia, into nitrite and nitrate, in the presence and with the consumption of oxygen.

Nitrate assimilation is a process by which inorganic nitrogen is converted to ammonia and then to organic nitrogen.