Which one of the following is a useful functional association between fungi and the roots of higher plants ?

The correct answer is Mycorrhiza because it is the specific mutualistic (functional) association between fungal hyphae and the roots of higher plants. The term ‘mycorrhiza’ literally denotes a fungus–root relationship and describes both ectomycorrhizal and endomycorrhizal types, in which the fungus and plant exchange nutrients and benefits, with hyphae often penetrating or enveloping root tissues to assist nutrient and water uptake [1]. These associations are widespread and enhance plant nutrition and stress tolerance by effectively extending the absorptive surface of the root system via fungal hyphae, functioning as living extensions of roots to access otherwise unavailable resources. This distinguishes mycorrhiza from the other listed options.

Sources

1. [1] https://www.sciencedirect.com/topics/immunology-and-microbiology/mycorrhiza