Why is the restriction enzyme kept cold during reaction setup?

Correct answer: To maintain enzyme stability

The governing laboratory principle is preservation of protein structure and enzymatic activity. Restriction enzymes are proteins, and many of them can lose activity when exposed for too long to room temperature, repeated warming, or unsuitable handling. Keeping the enzyme on ice during setup slows unwanted degradation and helps preserve its correct three-dimensional structure until the reaction begins. The reaction is later incubated at the enzyme’s specified working temperature, so cold storage is for handling and stability, not for performing digestion itself. Option A is therefore correct. DNA is not being frozen, buffer need not become solid, and agarose preparation is unrelated to restriction-enzyme handling.