Proteins are formed by condensation of 

• Proteins are polypeptides and are called building blocks of the body.
• They are made up of amino acids.
• Two amino acids undergo condensation reaction to form a dipeptide.
• Further condensation reactions result in a polypeptide.
• The amino acid units are linked by peptide bonds or peptide linkages.
  
• So, the correct answer is 'Amino acids'.