The source of electricity in torch is
cell
Explanation for correct answer - a cell or battery in a circuit produces power and is the source of electricity
Hence option (d) is the correct answer
Explanation for incorrect option
option (a)- a switch is used to make and break circuits but cant be a source of power
option (b)-Electricity to operate devices is obtained from mains (i.e)electricity is produced from power stations are transferred via overhead cables/transformers to mains .
From the mains it is routed to devices but not the source of electricity in a circuit
Hence option (b) is incorrect answer.
Option (c)- bulb is an electrical appliance that produces light.