The change of NH+4 to NO-3 is called as

Answer: Option A

The change of NH⁺₄ to NO^-₃ is called Nitrification.
Nitrification is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria.
The transformation of ammonia to nitrite is usually the rate-limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil. Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea.