The dark reaction of photosynthesis involves the reduction of carbon dioxide to organic compounds. The dark reaction, also known as the Calvin cycle, occurs in the stroma of the chloroplasts in plants. During this process, carbon dioxide is reduced by a series of enzyme-controlled reactions that use energy from ATP and NADPH, which are produced during the light-dependent reactions of photosynthesis.
In simpler terms, the dark reaction uses the energy produced from the light reaction to convert carbon dioxide into glucose and other organic compounds that the plant can use for energy and growth. Unlike the light reaction, the dark reaction does not require light to occur and can happen in the absence of light.
Therefore, the correct answer is option A: reduction of carbon dioxide to organic compounds.