Q.In the association seen in a lichen (a fungus and an alga/cyanobacterium living together), the interaction is best classified as:
(A) Commensalism
(B) Mutualism
(C) Parasitism
(D) Amensalism

Neither partner is harmed, ruling out parasitism, and both benefit, ruling out commensalism.

In a lichen, the fungal partner provides physical structure, anchorage, and absorbs water and minerals, while the algal or cyanobacterial partner photosynthesises and shares sugars with the fungus — both partners clearly benefit from the association, which is the defining feature of mutualism (+, +). Commensalism (option A) would require one partner to be unaffected, which is not the case here, since the fungus actively depends on the food the alga provides. Parasitism (option C) would require one partner to be harmed, which does not describe the lichen relationship. Amensalism (option D) requires one species to be harmed and the other unaffected, which also does not apply.

Check whether BOTH partners clearly benefit — if yes, it's mutualism; if only one benefits and the other is unharmed, it's commensalism; if one is harmed, it's parasitism or amensalism.

Do not select commensalism — a common error is assuming the fungus merely "hosts" the alga without benefiting; in fact the fungus actively depends on the sugars the alga/cyanobacterium produces, so both genuinely gain.