The correct symbol for representing a decision in a flow chart is a diamond. This shape is typically used to illustrate points in the program where a yes/no question is being assessed, making it fundamental for branching logic. In this scenario, whether the user inputs '1' or '2' is the branching condition. The rectangle represents a process step, not a decision, so it is not correct. The circle is generally used as a connector or terminator and the arrow represents the flow of control, not a decision.