The Calvin cycle, also known as the C3 cycle, is a series of biochemical reactions that occur in the chloroplasts of plants during photosynthesis. It is responsible for converting carbon dioxide (CO2) into glucose, which is used as an energy source for the plant.

In C4 plants, such as maize and sugarcane, an additional step is added before the Calvin cycle to increase the efficiency of CO2 fixation. This step involves the conversion of CO2 into a four-carbon compound called oxaloacetate, which is then transported to the bundle sheath cells.

The bundle sheath cells are a specialized type of cell found in the leaves of C4 plants. They are located around the veins of the leaf and are surrounded by mesophyll cells. The bundle sheath cells contain chloroplasts, where the Calvin cycle operates.

Now, let's look at the options given:

a) Stroma of bundle sheath chloroplasts: This option is correct. The Calvin cycle operates in the stroma of the chloroplasts in the bundle sheath cells of C4 plants. The stroma is the fluid-filled space within the chloroplasts where the enzymes involved in the Calvin cycle are located.

b) Grana of bundle sheath chloroplasts: This option is incorrect. Grana are stacks of thylakoid membranes within the chloroplasts, and they are primarily involved in the light-dependent reactions of photosynthesis, not the Calvin cycle.

c) Grana of mesophyll chloroplasts: This option is incorrect. Mesophyll cells are the cells surrounding the bundle sheath cells in C4 plants, and they contain chloroplasts where the initial CO2 fixation occurs. However, the Calvin cycle does not take place in the grana of these chloroplasts.

d) Stroma of mesophyll chloroplasts: This option is incorrect. While the initial CO2 fixation occurs in the stroma of the chloroplasts in the mesophyll cells of C4 plants, the Calvin cycle itself takes place in the stroma of the chloroplasts in the bundle sheath cells.

In summary, the correct answer is option 'A', as the Calvin cycle operates in the stroma of the bundle sheath chloroplasts in C4 plants.