What must be true for a transition to occur in a state transition diagram?

For a transition to occur in a state transition diagram, it is essential that a guard condition is satisfied. Guards are specific conditions or constraints that must be met for a transition from one state to another to take place. They serve as a way to control the flow of states based on certain criteria, ensuring that the system only moves to the next state when appropriate conditions are true. This concept is crucial in modeling the behavior of systems, as it allows for conditional logic that reflects real-world scenarios where certain actions can only happen under specific circumstances.

The other options do not represent necessary conditions for transitions. Displaying an error message is typically a result of a transition or an event but is not a prerequisite for a transition to occur. Initializing the state is important at the start-up of the system but does not pertain to the transition process itself. Similarly, the system being idle is not a requirement for a transition, as transitions can occur in response to various events regardless of whether the system is currently idle or active. Guard conditions specifically define when a transition can happen, making them the fundamental requirement in a state transition diagram.