Final answer:
The dark reactions, also known as the Calvin cycle, take place in the stroma of the chloroplast. They do not require light and utilize ATP and NADPH to convert carbon dioxide into glucose.
Step-by-step explanation:
In plants, carbon dioxide (CO2) enters the leaves through stomata and diffuses until it reaches the mesophyll cells. Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast—the site of the light-independent reactions. These reactions are sometimes called the Calvin cycle after the scientist who discovered them. The Calvin cycle takes place in the stroma and does not require light. Instead, it utilizes the ATP and NADPH produced during the light-dependent reactions to convert CO2 into glucose (carbohydrates).
The dark reactions, also known as the Calvin cycle or the light-independent reactions, take place in the stroma of the chloroplast. These reactions do not require light directly, but they are dependent on the products (ATP and NADPH) of the light-dependent reactions which occur in the thylakoid membranes. Therefore, the correct answer to the question is a) Carbon dioxide, as it is fixed during the Calvin cycle to produce sugars like glucose.