in the circuit shown, when the circuit is completed, the hammer strikes the gong. which of the following force is responsible for the movement of hammer?

The hammer moves due to magnetic force alone.

When the circuit is completed in an electric bell, current flows through a coil that acts as an electromagnet, creating a magnetic field that pulls the iron armature (with the hammer) towards the gong. This attraction is a magnetic force, not gravitational, electrostatic, or frictional.