The answer is D.regeneration
The Calvin Cycle is characteristic for photosynthetic organisms. It consists of three phases - carbon fixation, reduction, and regeneration. In the second phase, reduction, 3-phosphoglyceric acid (PGA) is reduced to glyceraldechide-3-phosphate. Further, in the third phase, regeneration, some of G3P are removed from the Calvin Cycle to be converted to glucose and other carbohydrates.