Question

Protein is a primary building block of ____ muscle

Answer 1

All of the above. :) 

Answer 2

Proteins are the main building blocks for tissues including muscle, hair, nails, skin, eyes, internal organs, nerves, ligaments, tendons, cartilage and membranes. They are also the building blocks of cells, cellular membranes, organelles--all the way to the level of DNA and genes.