The initial enzyme of the Calvin cycle is

Explanation

RuBisCO is the correct enzyme because it catalyses the first step of the Calvin cycle by fixing carbon dioxide to ribulose-1,5-bisphosphate, forming two molecules of 3-phosphoglycerate. This marks the beginning of carbon fixation in photosynthesis. Triose phosphate dehydrogenase is involved later in the cycle, helping convert 3-phosphoglycerate into triose phosphates using ATP and NADPH, but does not initiate the process. Cytochrome oxidase functions in cellular respiration, not photosynthesis, as it transfers electrons to oxygen in mitochondria. Hexokinase is an enzyme of glycolysis that phosphorylates glucose to glucose-6-phosphate and has no role in the Calvin cycle. Therefore, only RuBisCO is responsible for starting the Calvin cycle.