If a circular plasmid has two different restriction sites and both are cut, how many DNA fragments form?

Correct answer: Two

For a circular DNA molecule, the number of fragments produced by complete digestion generally equals the number of distinct cut positions, provided the cuts are at different sites. Here there are two different restriction sites and both are cleaved. The two cuts divide the circular DNA ring into two separate arcs, so two DNA fragments form. Option A is therefore correct. The one-fragment answer would describe an uncut circular molecule or an inappropriate assumption, while zero and five have no basis in the stated number of cuts. This counting rule is useful in restriction mapping, although repeated sites or incomplete digestion would change the observed pattern.