Answer :
The mutation of RuBisCO leading to its loss of function would impact the Calvin cycle by preventing the production of 3-PGA and consequently decreasing the production of G3P. Correct option is d.
If the enzyme RuBisCO is mutated such that it does not function, it would have a significant impact on the Calvin cycle, particularly on the initial stage of carbon fixation. RuBisCO is responsible for the critical step of adding CO2 to ribulose bisphosphate (RuBP), which results in the formation of 3-phosphoglycerate (3-PGA).
Without functional RuBisCO, this carbon fixation step would not occur, leading to a failure to produce 3-PGA and subsequent molecules downstream in the cycle, including glyceraldehyde 3-phosphate (G3P).
The correct answer is 'd. G3P production would decrease.' This is because G3P is a product of the reduction phase of the Calvin cycle, which cannot happen without the 3-PGA produced by the carbon fixation phase catalyzed by RuBisCO. Reduction and regeneration of RuBP would also be compromised, as they are dependent on the products of the carbon fixation phase.