Concept:Safety first: isolate the electricity supply before checking or repairing any faulty electrical fitting.Explanation:If a socket is faulty, the very first action is to cut off the main electrical power to prevent electric shock.Switching off only the socket may not be enough if the fault is inside. Removing the cover or checking wires without cutting the mains is dangerous.Therefore, switch off the mains before doing anything else.Answer:C. switch off the mains.