(d) Guanidine
Guanidin is not present in nucleic acid and as cytosine, Adenine, Thymine and Guanine are the four main bases found in nucleic acid.
Adenine and Guanine are purines while thymine and cytosine are pyrimidines. They together form triple bonded histones of DNA.
Guanidine is another compound which has a chemical formula HNC.
