On roads with heavy traffic, may the horn be used?

On roads with heavy traffic, may the horn be used?
A)Yes, but only outside built-up areas.
B)Yes, but only inside built-up areas.
C)No, except when strictly necessary.Correct
Drivers of non-priority vehicles may only use acoustic signals or warnings when strictly necessary and not for any purposes other than those permitted by regulations.
Their use is permitted to avoid an accident, to warn outside built-up areas of the intention to overtake, or to alert other road users of one's presence when carrying out an emergency service.