The hyphae of arbuscular mycorrhizal fungi (AMF) form bushy structures after making contact with the plasma membrane of a root cell. What is the function of these structures? a. They anchor the fungus inside the root, so the association is more permanent. b. They increase the surface area available for the transfer of nutrients. c. They produce toxins that protect the plant cells against herbivores. d. They break down cellulose and lignin in the plant cell wall.

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1. Step 1: Understand the question. The question is asking about the function of the bushy structures formed by the hyphae of arbuscular mycorrhizal fungi (AMF) after making contact with the plasma membrane of a root cell.
2. Step 2: Recall what you know about the relationship between AMF and plants. AMF form a symbiotic relationship with plants, where the fungi help the plant absorb nutrients from the soil, and in return, the plant provides the fungi with carbohydrates.
3. Step 3: Consider the options given. Option a suggests that the bushy structures anchor the fungus inside the root, making the association more permanent. While this might be true, it doesn't fully explain the main function of these structures. Option c suggests that the structures produce toxins to protect the plant cells against herbivores, but this is not a known function of AMF. Option d suggests that the structures break down cellulose and lignin in the plant cell wall, but this is not a function of AMF, rather it's a function of some other types of fungi.
4. Step 4: Choose the correct option. Option b suggests that the bushy structures increase the surface area available for the transfer of nutrients. This aligns with what we know about the symbiotic relationship between AMF and plants, where the fungi help the plant absorb nutrients from the soil. Therefore, option b is the correct answer.
5. Step 5: Review the answer. The bushy structures formed by the hyphae of AMF increase the surface area available for the transfer of nutrients between the fungi and the plant.