What is the function of agarose gel in gel-electrophoresis ?

The function of agarose gel in gel electrophoresis is to help separate DNA fragments based on their size through a sieving effect. Hence, the correct option is 1:

DNA fragments separate (resolve) according to their size through sieving effect.

Agarose gel, a natural polymer extracted from sea weeds, forms a porous matrix through which the DNA molecules can migrate when an electric field is applied during gel electrophoresis. The smaller DNA fragments move more easily through the gel pores, while larger fragments face more resistance and move more slowly. As a result, the DNA fragments get separated into distinct bands on the gel based on their size, with the smaller fragments traveling farther from the starting point.