Answer : (C) Thymine

Step 1 — List the bases in each nucleic acid

DNA : Adenine, Guanine, Cytosine, Thymine

RNA : Adenine, Guanine, Cytosine, Uracil

Step 2 — Identify what differs

Three bases — adenine, guanine and cytosine — are common to both. This rules out options (A), (B) and (D).

The fourth base is where they part company :

• DNA has thymine
• RNA has uracil in its place

Step 3 — Note the structural relationship

Thymine is simply 5-methyluracil — uracil with an extra methyl group at C-5. The two are otherwise identical, which is why either can pair with adenine.

Step 4 — Remember the base pairing

In DNA :

$$\text{Adenine} = \text{Thymine} \quad \text{(two hydrogen bonds)}$$

$$\text{Guanine} \equiv \text{Cytosine} \quad \text{(three hydrogen bonds)}$$

In RNA the partner of adenine is uracil instead.

Step 5 — The full comparison