What is one of the main challenges researchers face with informal interviews?

The correct choice highlights one of the significant challenges faced by researchers when conducting informal interviews: the unpredictable nature of discussions. Informal interviews often lack a rigid structure, allowing conversations to flow more naturally and spontaneously. This unpredictability can lead to varied responses that may diverge from the initial topic of discussion, making it difficult for researchers to maintain focus on specific research questions or themes. The open-ended nature of informal interviews can elicit rich, qualitative data, but it also introduces the challenge of navigating tangential topics and ensuring that key points are adequately covered.

In contrast, maintaining participant anonymity relates to ethical considerations and is a challenge in many forms of research, but it is not unique to informal interviews. Ensuring that responses can be categorized pertains more to structured methods where quantifiable data is necessary, and relies on a level of uniformity that informal methods intentionally abandon. Lastly, reliance on predetermined questions is characteristic of structured or semi-structured interviews rather than informal ones, making this choice less relevant to the specific challenges posed by informal interviewing techniques.