You have a duty to act when:

The correct choice is based on the understanding of legal and ethical responsibilities in emergency situations. A duty to act is typically established when a person's job or role inherently requires them to provide assistance in emergencies. For example, certain professions like healthcare workers, lifeguards, and emergency responders are obligated to respond and provide help when necessary because it is part of their job description.

This responsibility ensures that individuals in these positions are prepared to assist others while facing emergencies, protecting them and allowing for prompt aid to those in need. In this context, the other choices may imply a sense of responsibility, such as stopping at an emergency scene or having taken a first aid course, but they do not legally establish a duty to act beyond the context of specific employment or professional agreements.