Is about making sure experimental and control groups are equivalent, and is about selecting participants from a population so that the selected group is representative of that population.

random assignment, a random sample

Explanation:

In an experiment which requires groups the groups are to be made sure that members of each group is selected at random. This is known as random assignment.

The selection of participants from a population randomly i.e., having no pattern is called random sampling and the sample is called random sample. The random sample is selected as such in order to be representative of the population.

i have to go with a for this one

a would be the answer