The correct answer is (d) Humus, living organisms, and soil particles.
Explanation: Topsoil is the uppermost layer of soil, and it is characterized by a high concentration of organic matter (humus), diverse microbial life, and various soil particles.
Why other options are incorrect:
(a) Humus and living organisms only: While humus and living organisms are key components of topsoil, it also contains soil particles.
(b) Humus and soil particles only: Topsoil is not just composed of humus and soil particles; it also harbors a vibrant community of living organisms.
(c) Humus, living organisms, and plants: While plants contribute to the formation of humus through decomposition, the term “plants” is too narrow as topsoil contains a much broader range of life beyond just plants.