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

The correct answer is Mycorrhiza. This is a symbiotic or mutualistic association where a fungus lives in or on the roots of a higher plant. The fungus helps the plant by increasing its ability to absorb water and essential minerals, particularly phosphorus, from the soil. In return, the plant provides the fungus with sugars produced through photosynthesis.

While the other options relate to biology, they do not fit this specific definition. Biofertilizers are a broad category of organisms that enrich soil nutrients. Coralloid roots are specialized roots found in Cycads that associate with nitrogen fixing cyanobacteria, not fungi. Lichens are a symbiotic association between a fungus and an alga or cyanobacteria, not the roots of higher plants.