Correct Option: B

The tautomer forms of nitrogen bases are immuno-tautomer instead of amino group (e.g. cytosine, adenine) or enol instead of the keto group (e.g. thymine, guanine). Tautomers are organic compounds that are interconvertible by a chemical reaction called tautomerization. The concept of tautomers that are interconvertible by tautomerizations is called tautomerism. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.