When the bases are present in their rare imino or enol states, they can form which of the folowing pair combination?
a. a: t
b. c: g
c. a: c
d. a: t and c: g
e. c: g and a: c

The correct answer is c.  A:C

Explanation:

Thymine and guanine are most stable in their keto form and adenine and cytosine are most stable in their amino form. Sometimes these bases undergo tautomeric shift(proton shift) which converts them into rare less stable form called tautomeric form which are imino and enol form.

In this rare form, these bases can pair with the base which are not complementary to them. Like adenine base pairs with cytosine and guanine base pairs with thymine when present in rare form.

When these tautomeric forms of nucleotides incorporated in DNA they may cause mutation in the DNA.

Therefore the correct answer is c.  A:C .

the backbone of the dna and rna compound is sugar phosphate.