The right answer is because DNA is identical in all cells of the body.
The nucleus of a cell of the human body contains two DNA chains, one of which comes from the father, the other from the mother. Each molecule of DNA is identical for all the tissues of the body. So we find in a hair bulb, a skin cell, a blood stain or a bone cell. DNA is unique to each individual. We can then compare them all and see if we have to deal with the same individual.