The correct option is B Both assertion and reason are true but the reason is not the correct explanation of the assertion
Mycorrhiza is a mutually beneficial or symbiotic association of a fungus with the root of a higher plant. The fungal partner absorbs nutrients for the plant and the plant provides food and shelter to the fungus. Hence, this association is useful for plants. The assertion is true.
All fungi do not show mycorrhizal association. Apart from symbiotic fungi, such as the ones belonging to genus Glomus, there are many free living fungi, which don’t show any symbiotic association. Hence, the reason is also true. But, the reason is not the correct explanation of the assertion.