A bacterium that converts NO2 to NO3 is:

Explanation

Nitrobacter is the correct answer because it is involved in the second step of nitrification, where it converts nitrite (NO2) to nitrate (NO3). This is an essential step in the nitrogen cycle, making nitrogen available to plants in a more accessible form. Rhizobium is involved in nitrogen fixation, Bacillus is known for endospore production, and Nitrosomonas converts ammonia to nitrite, not nitrite to nitrate.