The correct option is B Serum albumin
Serum albumin is found in the blood and hence also known as blood albumin. It is the most abundant protein in human blood. It is synthesized in the liver, is monomeric and water soluble.
RuBisCO is known as Ribulose bisphosphate carboxylase oxygenase and is the most abundant protein in nature. It is an enzyme involved in photosynthesis.
Cellulose is a polysaccharide of ꞵ-glucose. It is a homopolymer, and constitutes the main component of plant cell walls. It is not a protein.
Collagen is the most abundant protein in the animal world and also in the human body. It is found in bones, muscles, skin and tendons. It provides strength and structure to the body.