Final answer:
Mycorrhizal fungi profit from their symbiotic relationship with trees by receiving sugars which the trees produce through photosynthesis, and in turn, the fungi aid the trees in nutrient uptake from the soil.
Step-by-step explanation:
Mycorrhizal fungi profit from their interaction with trees primarily by receiving sugars from the trees. While trees carry out photosynthesis to produce their own food, they allocate some of the produced sugars to the mycorrhizal fungi. In return, fungi help the plant by extending its root system through their hyphae, allowing the tree to access nutrients from the soil that would otherwise be unavailable, particularly phosphorus, zinc, and copper. This biological interaction is crucial, especially in nutrient-poor soils.
The fungi benefit significantly, obtaining up to 20 percent of the total carbon accessed by plants. Mycorrhizae also play a protective role for the plant by serving as a physical barrier to pathogens, inducing defense mechanisms, and sometimes even producing antibiotic compounds.
Therefore, the correct answer is: a) They receive sugars from the trees.