In addition to the elements found in carbohydrates and lipids, proteins contain the element a. potassium b. sulfur c. oxygen d. nitrogen

d. nitrogen

Explanation:

There are 4 types of bio molecules named carbohydrates, lipids, proteins and nucleic acids. Carbohydrates and lipids are made up of macro elements like carbon (C), hydrogen (H) and oxygen (O). But proteins additionally have nitrogen (N) apart from having carbon (C), hydrogen (H) and oxygen (O). They have some sulphur also but only in minute concentration that too in only a few peptides which have sulphur containing amino acids like methione and threonine An amine group (NH2) which contains nitrogen and forms amide bond is an absolute necessity for existence of a protein. An amide bond form backbone of a protein molecule.  

So the most suitable option is nitrogen.

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