.......... are the most abundant protein in the living world.

RuBisCO is the most abundant enzyme which is used to fix carbon during dark reaction of the photosynthesis. It is a primary acceptor of carbon dioxide during Calvin cycle. It is also known as Ribulose-1,5-bisphosphate carboxylase/oxygenase. It fixes the carbon dioxide to 3-phosphoglycerate. It also accepts oxygen during photorespiration and forms phosphoglycolate and 3-phosphoglycerate.

Collagen is an amino acid protein. It is a structural protein. It is mostly found in tendon, ligament, and skin. 90% collagen is present in the animal cells.

So, the correct answer is option B.