Which one of the following bases is not present in DNA?

We begin by recalling the basic information about the nitrogen-containing heterocyclic bases that make up the genetic material DNA. DNA contains two kinds of nitrogen bases:

1. Purine bases, namely $$\text{Adenine}$$ and $$\text{Guanine}$$.

2. Pyrimidine bases, namely $$\text{Cytosine}$$ and $$\text{Thymine}$$. (In RNA, $$\text{Uracil}$$ takes the place of $$\text{Thymine}$$.)

Thus, the complete, universally accepted list of the four bases actually present in DNA is

$$\{\text{Adenine},\; \text{Guanine},\; \text{Cytosine},\; \text{Thymine}\}.$$

Now we examine each option offered in the question and compare it with the list above.

Option A gives $$\text{Quinoline}$$. Quinoline is a fused bicyclic aromatic compound consisting of a benzene ring fused with a pyridine ring; it is not one of the canonical nitrogen bases of DNA.

Option B gives $$\text{Adenine}$$. From the accepted list we have just written, Adenine is definitely present in DNA.

Option C gives $$\text{Cytosine}$$. Our list likewise confirms that Cytosine is present in DNA.

Option D gives $$\text{Thymine}$$. Again, the list shows that Thymine is present in DNA.

Therefore, among the four choices, the only compound named that does not belong to the standard set of DNA bases is $$\text{Quinoline}$$.

Hence, the correct answer is Option A.