The correct answer is C) 75%. According to 46 CFR Part 54, which covers pressure vessels, steam safety and relief valves must be provided with a substantial lifting device capable of lifting the valve disc from its seat at 75% of the set pressure. This ensures that the valve can be properly tested and verified to be functioning correctly at pressures below the full set pressure. The other answer choices are incorrect - 0% would not provide any meaningful testing capability, 25% is too low, and 90% is too high and may damage the valve during testing.