Starting-air receivers on a reversible main diesel engine are required to hold enough air for how many consecutive starts before recharging, according to standard practice?

Why C is correct

The receivers are required to hold enough air for a specified number of consecutive starts, commonly twelve for a reversible main engine, before recharging is required. This reserve capacity ensures the engine can be maneuvered through repeated ahead and astern movements without waiting for the compressors to recharge.