Which device allows current to flow in only one direction?

The correct answer is a diode. A diode is a semiconductor device that allows current to flow through it in only one direction. This characteristic is fundamental to many electronic circuits, where controlling the direction of current flow is crucial for functionality.

Diodes conduct when the voltage at the anode (positive side) exceeds that at the cathode (negative side), allowing current to pass. Conversely, they block current flow when the voltage at the anode is lower than at the cathode. This property makes diodes essential components in rectification processes, where alternating current (AC) must be converted to direct current (DC).

Other devices listed do not exhibit this unidirectional current flow feature. For example, a transformer is used to change the voltage level of alternating current, but it does not restrict current flow direction. An electrical interlock is a safety device designed to prevent operation under unsafe conditions and does not affect the direction of current flow. Fuses serve to protect circuits by breaking the circuit in the event of excessive current, but they also do not control the direction of current.