The correct answer is C) when the generating capacity of the boiler is increased. According to U.S. Coast Guard regulations, the relieving capacity of boiler safety valves must be checked when the generating capacity of the boiler is increased. This is because an increase in boiler capacity can lead to higher steam pressures, which may require adjustments or replacement of the safety valves to ensure they can properly relieve the increased pressure. The other options, checking the valves annually or every 4 years, or only when repairs have been made, are not the specific regulatory requirements for this scenario.