The nucleic acid base having two possible binding sites is:

Guanine

The answer is (2) guanine.

Guanine has two possible binding sites because it can form hydrogen bonds with both cytosine and thymine. Cytosine and thymine can only form hydrogen bonds with guanine, so they only have one possible binding site. Adenine can only form hydrogen bonds with thymine, so it also only has one possible binding site.

Therefore, the only option that is correct is (2).

Here is a table showing the possible binding sites for each nucleic acid base: