Why do some dna fragments move farther than others during gel electrophoresis ?

The DNA fragments having shorter size than other move faster than other.

Gel electroscopes is a technique used to separate DNA fragments according to their size. DNA samples are loaded into wells  at one end of a gel, and an electric current is applied to pull them through the gel. DNA fragments are negatively charged, so they move towards the positive electrode. Hence short fragments having less number of nuecleotides move faster than long fragmentas.

Because their masses are different

i think the answer is b.