The correct option is A pH
Nitrification is an integral part of nitrogen fixation where the ammonia is oxidized to nitrite and nitrate, by the action of nitrifying bacteria. The soil pH is a known limiting factor of nitrification. Nitrifiers prefer a range of pH from 7 to 9. Nitrobacter can be slowed if there are toxic levels of NH3 and a pH greater than 7.5.