The correct option is D increasing its resistance to insects
Mycorrhizas are symbiotic relationships between fungi and plant roots. Such plant also show resistance to root pathogens but they do not show resistance to insects.
They are particularly important in assisting the host plant with the uptake of phosphorus which is vital to plant growth.
They also increase the surface area associated with the plant root, which allows the plant to reach nutrients and water that might not be available otherwise. Thus increasing its tolerance to drought.