Question

what object can be used at position x for the buzzer to sound when switch k is closed?
another bulb
another switch
a power source
an insulated wire

Explanation:

To make the buzzer (or bulb, as per the diagram) sound when Switch K is closed, a complete electric circuit is needed. A circuit requires a power source (like a battery) to provide electrical energy, conductors to carry current, and a load. Position X is part of the circuit that's missing the power source. Another bulb is a load, another switch is a control (not the energy source), and an insulated wire's insulation prevents current flow (we need a conductor, but the main missing element here is the power source). So a power source is needed at X to complete the circuit and allow current to flow when Switch K is closed.

Answer:

a power source