Answer:
Agarose gel or Pores in the gel
Step-by-step explanation:
This question could have multiple answers because the gel has pores in it that allow smaller fragments of DNA to move faster through the pores because they're not getting stuck in the gel. Larger fragments of DNA get stuck in the gel as they try to fit through the large pores.
So, you could either say the agarose gel itslef or you could say the pores in the agarose gel allow the molecules to move at different rates.