The correct answer is: B. A random sample.
A random sample is a subset of a population in which each member of the population has an equal chance of being selected. This means that the sample is representative of the population, and the results of the study can be generalized to the population as a whole.
A cluster sample is a type of sampling in which the population is divided into groups, or clusters, and then a random sample of clusters is selected. The members of each cluster are then included in the sample. This type of sampling is often used when it is difficult or expensive to collect data from the entire population.
A stratified sample is a type of sampling in which the population is divided into groups, or strata, based on one or more characteristics. A random sample is then selected from each stratum. This type of sampling is often used when it is important to ensure that the sample is representative of the population on the basis of the characteristics used to divide the population into strata.
A systematic sample is a type of sampling in which every nth member of the population is selected for the sample. This type of sampling is often used when it is easy to list the members of the population but difficult or expensive to collect data from each member.
In the given question, the researcher selects a probability sample of 100 out of the total population. This means that each member of the population has an equal chance of being selected. This is a random sample.