When can an emergency vehicle be passed on the left?

Answer: D — Never

An emergency vehicle should never be passed on the left in any situation. It is illegal to do so, as it poses a danger to both the emergency vehicle and other drivers on the road. Passing an emergency vehicle on the left could also impede their path and delay their response to the emergency they are attending to. Therefore, passing an emergency vehicle on the left is never a safe or acceptable option. Options A, B, and C suggest specific situations when passing an emergency vehicle on the left may be permissible, but this is incorrect and not in accordance with traffic laws. Answer D is the only correct and safe option.