What kind of DNA sample is needed for restriction digestion?

Correct answer: enzyme-compatible clean DNA / Enzyme-compatible clean DNA

The direct answer is option C: enzyme-compatible clean DNA. Restriction digestion means cutting DNA at particular recognition sites with restriction enzymes. These enzymes work only when the DNA sample is sufficiently pure and placed in a suitable reaction buffer. Step by step: first, DNA is isolated; second, proteins, phenol, alcohol, excess salt and other chemicals are removed; third, the clean DNA is mixed with the correct enzyme and buffer; fourth, the enzyme cuts at its specific site. Option A is wrong because protein contamination can block or reduce enzyme activity, so “always protein contaminated” describes a poor sample, not a required one. Option B is wrong because leftover ethanol can inhibit enzymes and interfere with the reaction. Option C is correct because clean DNA with conditions compatible with the enzyme allows reliable cutting. Option D is wrong because salt crystals alone are not a DNA sample and cannot be digested. A useful exam cue is: restriction enzymes need pure DNA, the correct buffer, proper temperature and enough time; contaminants are obstacles, not requirements.