Can someone explain the calvin cycle to me?

The Calvin Cycle is otherwise known as the light-independent part of photosynthesis. Here is what happens in stages: 

1) Ribulose biphosphate (RuBP) is a 5 carbon molecules that binds to a molecule of CO2 with the help of the enzyme Rubisco. This would theoretically make a 6 carbon compound but this is a very unstable compound that breaks down instantly to form 2 3 carbon molecules called glycerate 3-phosphate (GP). 

2) The 2 GP molecules are 'activated' by the use of an ATP molecule and are then reduced (due to the oxidation of NADPH to NADP) to 2 3 carbon molecules of triose phosphate (TP). 

3) The 2 TP molecules then lose 1 carbon atom (between them) and this carbon atom can be used to synthesise sugars. The remaining 5 carbon atoms bond together and are phosphorylated by a molecule of ATP back into the 5 carbon RuBP molecule so the cycle can start again. 

This means that every cycle produces one carbon atom. This means that 6 cycles are required to produce one molecules of glucose as glucose is a 6 carbon sugar. Note that this is why CO2 is in the overall equation for photosynthesis because the carbon from CO2 ultimately ends up in the sugars produced by the plant.

Ithink the answer is d.