Which of the following is a protein? ?
sample 1) adenine, cytosine, thymine, guanine
sample 2) adenine, cytosine, uracil, guanine
sample 3) carbohydrates
sample 4) amino acids

None of the above.

A protein is a large complex molecule made up of smaller units called amino acids which are attached to one another forming long chains or polymers.

Amino acids are the building blocks of protein but they are not proteins themselves.

During digestion, proteins are broken down right into their monomer units, and these are amino acids. The sequence of amino acids  determines each protein's unique 3 -dimensional structure and its specific function. 

Proteins do  most of the work in cells and are required for structure and function of cells.

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.