Lichens is an association of two types of organisms. They are

Answer: (a) Algae and fungi

A lichen is a fungus with algae (or cyanobacteria): the alga makes food, the fungus gives shelter and water.

The idea

A lichen looks like one plant but is two organisms living as one. The fungus forms the body, holds water and grips the rock. The alga lives inside, makes food by photosynthesis and shares it. Each gains, so the relation is mutualism.

Fungus: shelter, water, minerals → Alga: food from sunlight → Together: can live on bare rock, bark, even in deserts and polar lands

Also know

Lichens take everything from the air, so polluted air kills them; where they vanish, sulphur dioxide is high. Litmus, the dye used to test acids, comes from a lichen. “Reindeer moss” of the tundra is a lichen.