Which could be a source of emf in a circuit?

A. copper wire

B. light bulb

C. solar cell

D. switch

So here source of EMF in the given circuit is Solar Cell

Explanation:

EMF source is the energy source which provide constant potential difference across the conductor

So here we have

copper wire : this provide the path for the flow of electrons

light bulb : It provides the light energy in the circuit

Solar Cell : It provides the constant potential difference to the circuit

Switch : It is used to control the current flow in the circuit