How can vector backbone and insert be seen separately after restriction digestion?

Correct answer: By agarose gel electrophoresis

The direct answer is D: use agarose gel electrophoresis. After restriction digestion, the vector backbone and the insert are separate DNA fragments. Agarose gel contains a porous network. When an electric field is applied, negatively charged DNA moves through the gel, and smaller fragments usually travel farther than larger fragments. Step by step: digest the DNA; load the reaction into the gel; apply electricity; separate fragments according to size; stain or detect the DNA bands. If the insert and backbone differ sufficiently in length, they appear as separate bands that can be identified and, if needed, excised. Option D is correct. Option A is wrong because an ordinary microscope cannot reliably separate or identify DNA fragments of these sizes. Option B is wrong because smell cannot reveal DNA fragment size or identity. Option C is unsafe and scientifically irrelevant; ethanol should never be tasted, and taste cannot detect DNA fragments. Agarose gel electrophoresis is the standard visual method, although very similarly sized fragments may be difficult to distinguish.