Which of the following are highly useful in increasing soil fertility?

The correct option is B Bacteria

Bacteria increase the fertility of the soil by enriching the soil. It fixes nitrogen with the help of nitrogen-fixing bacteria and increases the nitrogen content of the soil. The nitrogen-fixing bacteria helps in the conversion of atmospheric nitrogen into salts of nitrogen which can be easily used by the plants. It takes place in presence of an enzyme known as nitrogenase enzyme. The bacteria also makes the soil porous which increases the aeration in the soil and improves its quality. Bacteria decompose the dead organic matter present in the soil and convert it to humus which further increases the fertility.

So, the correct answer is option C.