What factors will slow down the migration of solutes in electrophoresis?

The correct choice, which highlights that increasing the ionic strength of the buffer slows down the migration of solutes in electrophoresis, is based on the principles governing electrophoretic mobility.

When the ionic strength of the buffer increases, there is a greater concentration of ions in the solution. These additional ions can shield the charges on the solutes being separated, reducing their effective charge and, consequently, their mobility through the gel or medium. Therefore, higher ionic strength leads to greater electrostatic interactions and a more complex environment, which hinders the movement of solutes.

In contrast, decreasing the viscosity of the medium would generally enhance the mobility of solutes, allowing for faster migration as they encounter less resistance moving through the medium. Increasing the size of the molecule can indeed slow down migration due to increased frictional resistance, but it does not encompass the entirety of factors that can influence migration rates, particularly in the context of changes to the medium itself.

The option involving both increasing viscosity and decreasing ionic strength does not directly relate to common electrophoretic practices since typically, a decrease in ionic strength would also lead to less shielding of charges, thereby allowing for greater migration of charged particles.