Which of the following is an example of a user action that could trigger an event handler?

A keyboard press is a clear example of a user action that can trigger an event handler because it directly involves a user's interaction with the application. Event handlers are designed to respond to specific actions initiated by the user, and a keyboard press is one of the most common interactions. When a key is pressed, the event handler linked to that action can execute predefined code, such as submitting a form, moving to the next input field, or performing a search.

Other scenarios listed, while they may involve events, do not stem from direct user actions. A system prompt timeout is system-driven and does not require any user input to occur. A scheduled task completion is also event-driven but pertains more to automated processes rather than user interactions. Similarly, a network response results from server communication rather than direct user actions, making them unsuitable examples of user-triggered events.