Which one of the following is a research tool?

The correct answer is C. Questionnaire.

A questionnaire is a research tool that is used to collect data from a group of people. It is a list of questions that are asked to the participants, and the answers are used to answer the research question.

A graph is a visual representation of data. It is used to show the relationship between two or more variables.

An illustration is a drawing or painting that is used to explain or clarify something.

A diagram is a drawing that shows the structure of something.

All of these tools can be used in research, but a questionnaire is the most common type of research tool.