First phase of Calvin cycle is:

Explanation

The first phase of the Calvin cycle is the carbon fixation phase, which makes option A correct. In this phase, carbon dioxide from the air reacts with ribulose-1,5-bisphosphate through the enzyme Rubisco to form two molecules of 3-phosphoglycerate. This step marks the beginning of the cycle by capturing inorganic carbon and turning it into an organic compound. The reduction phase, option B, comes after carbon fixation and uses ATP and NADPH to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate, so it is not the first phase. The regeneration phase, option C, occurs at the end of the cycle and uses ATP to rebuild ribulose-1,5-bisphosphate, allowing the cycle to continue. The photolysis phase, option D, does not belong to the Calvin cycle at all; it takes place during the light reactions, where water is split to release oxygen and provide electrons. Therefore, only the carbon fixation phase correctly represents the first step of the Calvin cycle.