Answer: A key, a conducting wire, a battery/cell , a resistance.
Explanation:
A circuit is a loop through which electric current flows. Thus, a conducting wire is essential.
An energy source is required-one which provides a potential difference across the two terminals. A cell or a battery can be used.
A key is required to open or close the circuit. Current flows only when the circuit is closed i.e. it forms a loop.
A component which is required to work - A resistance - A bulb or a motor etc.