In an observational study of children in a day care setting, you select one participant at a time for observation. This is an example of using .

individual sampling

Explanation:

Individual sampling: In psychology, the term "individual sampling" is described as a process in which each member of a specific population consists of a known, and a "non-zero chance" of being considered or selected as one of the different parts of the given sample. However, there are various kinds of individual sampling.

In the question above, the given statement represents individual sampling.

is there a year set? the question is incomplete

answer; the president uses mass media to gain support for policies;