The correct option is A Both A and R are correct and R is the correct explanation of A.
Mycorrhizae is the symbiotic association of fungus with the root system. Fungal filaments forms network around the young roots/penetrate the roots and increase the amount of water absorption in these plants. Mycorrhizae are particularly important in assisting the host plant with the uptake of phosphorus and nitrogen, two nutrients vital to plant growth. Mycorrhizae actually increase the surface area associated with the plant root, which allows the plant to reach nutrients and water that might not be available otherwise.