How does a plant benefit from an ociation between the plant's roots and a fungus?

The answer is (d.) for sure. Mycelium is vital in ecosystems for its role in the decomposition of plant material. The mycelium of mycorrhizal fungi increases the efficiency of water and nutrient absorption of most plants and confers resistance to some plant pathogens. First, the hyphae secrete enzymes onto or into the food source, which break down biological polymers into smaller units such as monomers. These monomers are then absorbed into the mycelium and most plants by facilitated diffusion and active transport.