Why is a restriction-cut vector purified before ligation?

Correct answer: To remove residual buffer, enzyme, salts, and unwanted small DNA fragments

After restriction digestion, the vector preparation may contain enzyme, salts, buffer components, glycerol, undigested molecules, and small unwanted fragments. Some of these substances can interfere with DNA ligase activity or with the correct joining of vector and insert. Purification, often by a cleanup column or gel extraction when size selection is required, gives cleaner DNA and can remove an unwanted small fragment or the original undigested vector. The purified vector ends are then supplied to the ligation reaction in a more suitable buffer and concentration. Option A is correct. Purification does not make a vector produce protein, turn agarose into water, or permanently activate a restriction enzyme, so B, C, and D are incorrect.