Why is it important that the
substance dna is placed in for
electrophoresis be porous?

It important that the  substance DNA is placed in for  electrophoresis be porous because,

DNA are placed in the electrophoresis gel, so that the pores present can limit or check the movement of the DNA by creating a surrounding in which individually DNA fragment rate of motion can vary based upon their length. The gel is nothing but a permeable matrix. The motion of charged molecule defined as the migration. The molecules migrate towards the opposite charge. The molecule having negative charge will be attracted towards the positive charge.

it is a. i hope this .