Final answer:
In a Common Base (CB) transistor circuit, the Emitter is the common terminal for both input and output signals, functioning as a junction for the flow of current. The correct option in 3.
Step-by-step explanation:
In a Common Base (CB) transistor circuit, the terminal that is common to both the input and output signals is the Emitter. When a base voltage is applied, it acts like a valve controlling the electric current that flows from the emitter to the collector. In this configuration, the base acts only as a control element, and hence the emitter is the common terminal shared between the input and output.