Why is buffer necessary in a restriction digestion mixture?

Correct answer: To provide proper pH and salt conditions for the enzyme

A restriction-digestion buffer supplies the chemical environment required for the selected restriction enzyme to function. Its pH helps preserve the enzyme’s charge, folding, and active-site configuration, while salts control ionic strength and often provide essential cofactors such as magnesium ions needed for phosphodiester-bond cleavage. If the pH, salt concentration, or cofactors are unsuitable, digestion may be slow, incomplete, nonspecific, or absent. Therefore option C is correct: the buffer provides suitable pH and salt conditions for the enzyme. It does not stain DNA, transform protein into starch, or promote cell growth; those activities are unrelated to an in-vitro DNA-cleavage reaction.