The correct answer is: A. smaller
Permittivity is a measure of how easily an electric field can be established in a material. A material with a high permittivity will allow an electric field to be established more easily than a material with a low permittivity. This means that the potential gradient across a material with a high permittivity will be smaller than the potential gradient across a material with a low permittivity.
To understand this, consider the following analogy. Imagine that you are trying to push a charged particle through a material. The material with a high permittivity is like a very thick fluid, while the material with a low permittivity is like a very thin fluid. It will be much easier to push the charged particle through the thin fluid than through the thick fluid. This is because the thin fluid offers less resistance to the motion of the charged particle.
In the same way, an electric field will be able to establish itself more easily in a material with a high permittivity than in a material with a low permittivity. This is because the material with a high permittivity offers less resistance to the establishment of the electric field.
As a result, the potential gradient across a material with a high permittivity will be smaller than the potential gradient across a material with a low permittivity.