The process of selecting elements from a population, collecting data from them, and using it as representative of all those a researcher is interested in is referred to as

The answer is sampling. It is the process of selecting units (like from a group of people, organizations) from a population of interest so that by studying the sample we may fairly simplify our results back to the population from which they were selected. A sample is the set of people who take part in the research. The people who opt in are referred to as "participants".