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

1. The question asks to identify a useful functional association between fungi and the roots of higher plants. 2. (A) Biofertilizer: This is a broad term for substances containing living microorganisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. Some fungi act as biofertilizers, including those forming mycorrhizae. 3. (B) Coralloid root: These are specialized roots found in cycads that are associated with nitrogen-fixing cyanobacteria, not fungi. 4. (C) Lichen: This is a symbiotic association between fungi and algae or cyanobacteria, typically growing on surfaces like rocks or trees, not directly with the roots of higher plants. 5. (D) Mycorrhiza: This term literally means 'fungus-root' and refers specifically to the symbiotic (usually mutualistic) association between a fungus and the roots of a vascular plant (higher plants). The fungus helps the plant absorb nutrients (like phosphorus) and water, while the plant provides carbohydrates to the fungus. This is a direct and important functional association.