Mycorrhizae enhance plant nutrition mainly by a. absorbing water and minerals through the fungal hyphae. b. providing sugar to root cells, which have no chloroplasts. c. converting atmospheric nitrogen to ammonia. d. enabling the roots to parasitize neighboring plants.

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1. Identify the primary function of mycorrhizae in plant nutrition. Mycorrhizae are symbiotic relationships between fungi and plant roots that are known to enhance plant growth and nutrient uptake.
2. Analyze each option in relation to the role of mycorrhizae: Option (a) suggests that mycorrhizae absorb water and minerals through fungal hyphae, which is a known function of mycorrhizal fungi in aiding plant nutrient absorption.
3. Evaluate option (b) which states that mycorrhizae provide sugar to root cells. This is incorrect as it is the plant that typically provides sugars to the fungi, not the other way around.
4. Consider option (c) which involves the conversion of atmospheric nitrogen to ammonia. This is a function of nitrogen-fixing bacteria, not mycorrhizal fungi.
5. Dismiss option (d) as it suggests that mycorrhizae enable roots to parasitize neighboring plants, which is not a function of mycorrhizal relationships; they are mutualistic, not parasitic.