A circular plasmid has four restriction sites. How many fragments will form after complete digestion?

Correct answer: Four

The direct answer is C: four fragments. For a circular DNA molecule, if there are four distinct restriction sites and digestion is complete, the four cuts divide the circular molecule into four separate arcs. Each arc becomes one DNA fragment, so the number of fragments equals the number of cuts. Option C is correct. Option A, three, would be expected for a linear DNA molecule with four internal cuts, because a linear molecule gives cuts plus one fragments. Option B, five, is also the linear-DNA rule applied incorrectly or an overcount. Option D, one, ignores the fact that four cuts separate the circle into multiple pieces; one fragment would describe an uncut plasmid, not complete digestion. This answer assumes all four sites are distinct, accessible, and actually cut by the enzyme. The useful rule is: circular DNA with n cuts gives n fragments; linear DNA with n cuts gives n+1 fragments. Memory cue: close the circle, so the last cut joins back to the first; four cuts make four pieces.