What symbol represents the initial state in a state transition diagram?

In a state transition diagram, the initial state is represented by a solid circle. This symbol is used to clearly denote where the state machine begins its operation. The solid circle serves as the starting point from which transitions to various states can occur, effectively indicating the entry point into the system's behavior.

In contrast, the other symbols have different meanings. A dashed circle typically represents an end state, while a rectangle is often used to illustrate states themselves. A triangle does not have a standard use in state transition diagrams. Thus, the solid circle's distinct design and universally accepted meaning as the starting state make it the correct choice for identifying where a state transition diagram commences.