The correct option is
C Calvin cycle
RuBP stands for ribulose bisphosphate, it's a 5 carbon compound involved in the Calvin cycle, which is part of the light-independent reactions of photosynthesis. Atmospheric CO2 is combined with RuBP to form a 6 carbon compound, with the help of an enzyme (biological catalyst) called RuBisCo. Hence the importance is that its part of the cycle that enables plants to 'fix' carbon from the atmosphere and convert into photosynthetic products (carbohydrate, proteins, fats). Since plants are the main autotrophic group on the planet and are at the bottom of most food chains RuBP is not just important to photosynthesis but to all heterotrophic life.
So, the correct option is 'Calvin Cycle'.