Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of

• (a) algae and bacteria: Incorrect. Lichens are not the standard symbiotic association of algae with bacteria.
• (b) algae and fungi: Correct. A lichen is a symbiotic association between a fungus and an algal partner (or cyanobacterium in some cases); in UPSC basics, the accepted answer is algae + fungi.
• (c) bacteria and fungi: Incorrect. Though cyanobacteria may occur in some lichens, lichens are classically defined as fungus + alga, not bacteria + fungi.
• (d) fungi and mosses: Incorrect. Mosses are separate bryophyte plants and do not form lichens with fungi.

Therefore, the correct answer is (b).