Which of the following statements is/are correct?

a. Lichens and mosses help in soil formation. b. Bits of decayed living organisms in the soil are called humus. c. Humus can be found in the subsoil.

This is the correct answer as these statements are correct. Lichens grow on the surface of rocks and while growing, they release certain substances that cause the rock surface to powder down and form a thin layer of soil. Mosses grow on this surface and they cause the rock to break up further. Bits of decayed living organisms in the soil are called humus.