What is a questionnaire used for in research?

A questionnaire is used primarily to collect data through written questions, which is why this option is the correct choice. In research, questionnaires are a structured method of inquiry that allows researchers to gather information from a large number of participants efficiently. They typically consist of a series of questions aimed at understanding respondents' opinions, behaviors, or experiences related to a specific topic. This method can be administered in various forms, such as paper-based, online, or face-to-face.

The other options do not accurately describe the function of a questionnaire. Conducting in-depth interviews focuses on qualitative data collection and involves open-ended questions meant to elicit detailed responses, which is distinct from the structured nature of a questionnaire. Analyzing existing data refers to secondary data analysis, where researchers examine previously collected data rather than gathering new data through questionnaires. Observing participants in their natural environment pertains to observational research, where researchers watch subjects in their real-world settings rather than relying on self-reported responses that a questionnaire would capture.