Which research method allows respondents to answer questions via email or web?

The correct choice is online survey, as this research method is specifically designed to collect responses from participants through digital platforms like email or the web. Online surveys enable researchers to gather large amounts of data efficiently and can include various types of questions, such as multiple choice, Likert scales, or open-ended responses. This flexibility allows for a wide range of insights into user preferences, experiences, and behaviors.

In contrast, usability testing typically involves observing users as they interact with a product in real-time, making it an in-person or controlled remote activity rather than a remote survey method. Focus groups require small groups of participants to discuss topics in a guided setting, usually in person or through videoconferencing, rather than completing an individual survey. Interviews, while they can be conducted online, often involve direct one-on-one interaction with a researcher, focusing on in-depth qualitative data collection rather than the breadth of information typically obtained from a survey format. Thus, online surveys stand out for their ability to reach a broad audience efficiently through email or web-based platforms.