Ag: t base pair in dna suggests that

(a) the t was originally a cytosine, which was deaminated to a t
(b) the t was originally a 5-methyl cytosine, which was deaminated to a t
(c) the t was originally a 5-methyl cytosine, which was oxidized to a t
(d) the g was originally an a, which was methylated to form an g

(A) The T was originally a cytosine, which was deaminated to a T

Explanation:

Adenine and guanine are found in both DNA and RNA.

Cytosine is found in both DNA and RNA.

Thymine is normally found in DNA.

In a CpG site (5'—C—phosphate—G—3') , that is, Cytosine and Guanine separated by only one phosphate group; phosphate links any two nucleosides together in DNA.

In a G:T base pair in DNA, Adenine pair with Thymine and Guanine pair with Cytosine.

Here in the question, The G:T base pair in DNA suggests that the T was originally a cytosine, which was deaminated to a T.

How methylation of CpG sites followed by spontaneous deamination leads to a lack of CpG sites in methylated DNA.

A  Cytosine base followed immediately by a Guanine base (a CpG) is rare in vertebrate DNA because the Cytosine in such an arrangement tend to be methylated. This methylation helps to distinguish the newly synthesized DNA strand from the parent strand, which aids in the final stages of DNA proofreading after duplication. However, over time methylated cytosines tend to turn into Thymine because of spontaneous deamination.