I would say
a. biological pest control
biological pest control is when other plants or animals are used to control pests. There are no chemicals used only the natural roles of the environment.
Hope this helped!
Answer: a. biological pest control
Explanation:
Biological pest control is the process of controlling the colonization of disease causing agent by a biological agent like insect, animal, microbe or other living being.
According to the given situation the nematodes are biological control agent which controls the growth of mole crickets. Therefore, biological pest control is the correct option.