When every individual in a large population has a small but equal chance of being included in a survey, researchers are using a procedure known as

Random sampling

Explanation:

In research, the term random sampling refers to the use of procedures involving just chance when creating groups for a study so that each participant has the same opportunity to be assigned to any group. In other words, the participants get to be randomly assigned to the groups and therefore this creates a more valid experiment. Since the participants are randomly assigned, this method helps ensure that the characteristics of the participants don't affect the results of the study (since they were randomly assigned)

Therefore, when every individual in a large population has a small but equal chance of being included in a survey, researchers are using a procedure known as random sampling.

c: in the east might be the correct answer