Answer:
The right answer is C.
Explanation:
Tautomeric shift type of mutation that happens spontanously by intramolecular proton (hydrogen ion) shift. Guanine and thymine normally found in their keto form but rarely they can be also found in enol forms. Moreover, while adenine or cytosine are normally found in amino forms they can rarely switch into their imino forms by intramolecular proton shift (see figure).
