A researcher decides to look at every fourth patient admitted each day and record if...

Detailed Explanation:

Systematic sampling involves selecting every nth individual from a list or sequence after a random start.

Option C: Systematic

By choosing every fourth patient after arandom start, the researcher is using systematic sampling.

Option A: Simple

Simple random sampling would involve selecting individuals at random, without a set interval.

Options B and D:

Convenience sampling is based on ease of access, and stratified sampling divides the population into subgroups before random selection.