Which of the following can be used to complete an electric circuit if one runs…

Materials fall into two broad electrical categories: conductors, which have freely moving electrons and allow electric current to pass through them, and insulators, which hold their electrons tightly and block current flow. To complete (close) a broken electric circuit, the substitute material must itself conduct current — it must bridge the gap the same way the missing wire would.

Checking each option against this conductor/insulator distinction:

• A paper strip is made of cellulose fibres, whose electrons are tightly bound rather than free to move — the defining trait of an electrical insulator.

• A rubber belt is a polymer whose electrons are tightly bound within the material — rubber is used to sheath current-carrying wires precisely because it is a well-known electrical insulator.

• A wooden stick is an organic material; dry wood's electrons are not free to move, which is the defining trait of an insulator rather than a conductor.

• A blade is made of metal, and metals are defined by a sea of delocalised electrons that move freely under an applied voltage — the defining trait of an electrical conductor.

Among the four materials, the blade fits the conductor definition (free-moving electrons); the other three fit the insulator definition (bound electrons).