The correct answer is A. Oxidation.
Biochemical treatment of sewage effluents is a process of using microorganisms to break down organic matter into simpler compounds. This process is essentially a process of oxidation, as the microorganisms use oxygen to convert the organic matter into carbon dioxide and water.
Dehydration is the process of removing water from a substance. This is not the goal of biochemical treatment of sewage effluents, as the goal is to remove the organic matter from the water, not the water itself.
Reduction is the process of gaining electrons. This is not the goal of biochemical treatment of sewage effluents, as the goal is to remove the organic matter from the water, not to add electrons to it.
Alkalinization is the process of increasing the pH of a substance. This is not the goal of biochemical treatment of sewage effluents, as the goal is to remove the organic matter from the water, not to change the pH of the water.