Which example represents non-verbal assent?

Non-verbal assent refers to a person's agreement or acceptance expressed through body language or other non-verbal cues rather than spoken words. In the context of the provided options, smiling while participating is a clear indication of non-verbal assent. A smile generally signifies that a person is comfortable, agreeable, or happy with a situation. This action can convey a positive response and show that the individual is engaged and willing to participate without the need for verbal confirmation.

The other choices include actions that do not imply agreement or acceptance. For instance, shaking one's head negatively indicates disagreement, and vocal affirmations are verbal in nature, thus not fitting the non-verbal category. Refusing to make eye contact can suggest discomfort or avoidance, which would also not signify assent. Therefore, smiling while participating best fits the definition of non-verbal assent.