The correct option is
B Reductive carboxylation
Calvin cycle is also known as C
3 cycle as it includes three steps carboxylation, reduction and regeneration of RuBP that leads to the formation of first three carbon compound 3-phosphoglyceric acid. The process involves the addition of carbon so it is carboxylation and addition of hydrogen that make it a reduction.
So, the correct option is 'Reductive carboxylation'.