In some cases, researchers are interested in very large groups such as all the inmates of the u. s. prison system. in this case, what would all of the inmates represent?

The population.

Explanation:
Practically, it is not usual to involve an entire population in a research study in order to draw conclusions or to make inferences.

For example, to make inferences about residents in the USA who go hiking will be impractical, if it requires questioning everyone in the USA.
Rather, a representative random sample is selected and questioned, and statistical techniques are utilized to make inferences and predictions about the population with confidence levels.