Answer: SAMPLE.
Explanation: He obviously cannot study every 12th grader, so instead he will select a smaller SAMPLE of seniors to study.
Sample in research methodology is defined as a subset of a population selected for measurement, observation or questioning, to provide statistical information about the population. A sample is referred to as an unbiased estimator of a population.
This marital practice is called : Endogamy
Endogamy is a marital process in which the grooms and brides only limited from within local community or tribe.
This marital method is deemed to be out-of-date and only carried out in the past to maintain the 'bloodline' of a specific tribe or clan.