An exploratory research on a small scale is called

An exploratory study carried out on a small scale before the main research is called a pilot test.

A sample test is not a standard term for a small preliminary study.

A pre-test usually checks a specific instrument such as a questionnaire, not the whole study design.

An informal test is not a recognised methodological category.

The pilot test uncovers practical problems so that procedures can be refined before full-scale work.

It also helps estimate feasibility, cost and time for the main study.