Which of the following protein is the most abundant in human body? 

The most abundant protein in the human body is collagen. Collagen is the main structural protein of the various connective tissues in animals. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendons, ligaments and skin. It is also abundant in corneas, cartilage, bones, blood vessels, the gut, and the dentin in teeth. In muscle tissue, it serves as a major component of the endomysium. 

So, the correct answer is option A.