The layer of soil that contains the most organic material is called the horizon.

The letter that best fits in the blank of the sentence, "The layer of soil that contains the most organic material is called the horizon" is O. This is the horizon with three distinct organic layers: one of pine needles, one of the leaves, and one of the twigs.

Most organic content is present in the top most layer, i.e., The O layer. It is rich in undecomposed plant residues. The O itself stands for the word organic. It is the surface layer and also contains material from dead plants and animals.It is mainly present in forests, and is also called as forest floor.

Ibelieve the answer is c.