Gel electrophoresis separates dna fragments on the basis of differences in their a: c ratio

The answer is length.

A gel electrophoresis is used for separation of DNA or its fragments on the basis of differences in their length. An electric current is running through the gel with positive pole on the bottom. Because of the difference in the lengths of DNA fragments, some of them will travel more quickly than other. It is expected to see smaller DNA fragments at the bottom of the gel and larger DNA fragments near to the top of the gel.