Final answer:
In a single-phase circuit, only one current responsive device is required.
Step-by-step explanation:
A single-phase circuit is an electrical circuit that carries an alternating current (AC) with only one phase. In a single-phase circuit, the voltage periodically changes direction, and the power is delivered in a single sinusoidal waveform. Single-phase circuits are commonly used in residential, commercial, and some small industrial applications.
In a single-phase circuit, only one current responsive device is required.
Typically, the current responsive device in a single-phase circuit is a breaker or a fuse. These devices are used to protect the circuit from excessive currents and prevent overloading.
Therefore, the correct answer is 1) One.