The correct answer is B) when the generating capacity of the boiler is increased. The reasoning behind this is that the relieving capacity of boiler safety valves must be checked whenever the generating capacity of the boiler is increased, to ensure the safety valves are adequately sized to handle the increased steam production. This is a requirement specified in the Coast Guard regulations. The other options are incorrect - option A refers to repairs, option C specifies a fixed 4-year interval, and option D specifies an annual check, which are not the correct regulatory requirements for checking the relieving capacity of boiler safety valves.