The correct answer is B) Steam pressure. In a turbo-generator's nozzle control speed regulator, the poppet valves are held closed by the steam pressure. The steam pressure acts on the poppet valves, creating a force that keeps them closed until the regulator needs to adjust the steam flow to maintain the desired generator speed. The other options, such as springs (A), a lifting beam (C), or oil pressure (D), are not typically used to hold the poppet valves closed in this type of regulator. The use of steam pressure is the standard mechanism for this purpose in turbo-generator nozzle control speed regulators.