Question

2.20 unit 2: biochemistry summative test
the element that is found in all proteins is
nitrogen
boron
argon
zinc

Explanation:

Proteins are made of amino acids. Amino acids have an amino group ($\ce{-NH2}$) which contains nitrogen. Nitrogen is a key element in all proteins. Boron is not a common element in proteins. Argon is a noble gas and not part of protein structure. Zinc may be in some proteins (like zinc - finger proteins) but not in all.

Answer:

Nitrogen