Out of A-T, G-C pairing, bases of DNA may exist in alternate valency state owing to arrangement called as

Nitrogen bases are aromatic cyclic ketones with amine or imine group. Tautomers are a type of structural isomers which are formed by shifting of pi-bonds between functional groups. They are also called as keto-enol isomers and are inter-convertible into each other as ?-C=C-C-OH? change to ?C-C-C=O? . Tautomerization converts the amino (-NH2) group of cytosine and adenine into amino (-NH) group and keto (C=0) of thymine and guanine into enol group (-OH).  This results in the pairing of tautomeric thymine with normal guanine and of cytosine with adenine. The resultant mutation is caused by tautomeric mutations.  Thus, the correct answer is option A.