A political polling firm randomly selects a sample of 50 potential voters form a list of registered voters in each county in a state to interview for an
upcoming election. What type of sample is this?

Stratified Random Sampling

Explanation:

Take note, we are told they randomly selected from "50 potential voters from a list of registered voters in each county in a state." This course of action by the polling firm follows exactly the Stratified Random Sampling type.

The Stratified Random Sampling method involves a deliberate division of a research population into strata or smaller sub-groups.

So in this case, the state was divided into strata or smaller sub-groups, that is, according to each county.