For nitrification, the optimum pH is

Nitrification is the microbial process by which reduced nitrogen compounds are sequentially oxidized to nitrite and nitrate. The bacteria involved in the process of nitrification are very sensitive to pH. The pH should be between 7.0 - 8.0 for Nitrosomonas, and the optimum pH range for Nitrobacter is approximately 7.5 - 8.5.
Therefore as per the given options, the optimum pH for the nitrification process is 8.5.