Answer:
In statistics, there are several strategies to select a sample. Remember that a sample is a sub group selected from a population. This sample selection must be random to have more reliability in the research, each answer option represents a type of sampling.
In this case, the researcher is using a multistage sampling, because it consists in taking smaller sample units at each stage, which is being done in this example. First, is selected a number of counties, then the precincts, and at the end the voters. By this way, the sample is being reduced to less subjects at each stage.