The correct answer is A. unweathered bedrock.
The R-horizon is the deepest soil horizon and is composed of unweathered bedrock. It is the parent material from which the overlying soil horizons have developed. The R-horizon is typically not considered to be part of the soil profile, as it is not actively involved in soil formation.
The B-horizon is the subsoil horizon and is composed of weathered parent material. It is darker in color than the A-horizon and has a higher clay content. The B-horizon is important for water storage and plant growth.
The A-horizon is the topsoil horizon and is composed of organic matter, minerals, and living organisms. It is the most fertile soil horizon and is the most important for plant growth.
The C-horizon is the intermediate soil horizon and is composed of weathered parent material that has not been significantly altered by soil formation processes. It is typically lighter in color than the B-horizon and has a lower clay content.
Plant litter is organic matter that has fallen to the ground from plants. It is composed of leaves, stems, flowers, and other plant parts. Plant litter is an important part of the soil ecosystem, as it provides food and shelter for soil organisms. It also helps to improve soil structure and fertility.
The correct answer is A. unweathered bedrock.