What occurs in the a horizon of soil?

a. parent material is weathered into smaller fragments.
b. the crystal structure of minerals in rock is broken down.
c. microorganisms decompose organic compounds into nutrients.
d. leached minerals from previous layers are precipitated out.

C. Microorganisms decompose organic compounds into nutrients.

Soil horizons are the layers of soil lies parallel to the surface of the earth crusts. The composition of the  each soil horizon varies on the basis of the amount of organic matter present in the soil, mineral composition, texture of soil, porosity of soil. The sequence of different layers of soil horizon is called as soil profile.

A soil horizon lies below the O soil horizon. It supports the high biological activity of soil microbes as organic matter obtained from the dead animals and plants gets decomposed by the microbes in the soil, to obtain nutrition from it.