Lichens

Lichens are defined in NCERT Class 11, Chapter 2 (Biological Classification) as symbiotic associations between an alga and a fungus, where the algal component is called the phycobiont and the fungal component the mycobiont. In this mutually beneficial relationship the autotrophic alga performs photosynthesis and prepares food for the association, while the fungus provides shelter, absorbs water and mineral nutrients, and anchors the body to the substratum. Therefore, preparation of food is the algal contribution. Absorption of water and minerals is the role of the fungal partner, not the alga, so that option is incorrect. Providing the protective thallus body is again a fungal function, since the bulk of the lichen structure is fungal. Anchoring to the rock is performed by fungal outgrowths, not by the alga. Lichens are also valued as excellent pollution indicators because they fail to grow in polluted areas. A logical consistency check supports the answer: only the alga possesses photosynthetic pigments, so food manufacture must necessarily be its specific contribution, while the heterotrophic fungus depends on it for nourishment in exchange for protection and water.