The correct answer is D) Open the pump suction valve prior to admitting steam to the turbine. This is the correct answer because it is important to ensure that the pump suction is primed and there is adequate supply of feed water to the pump before starting the turbine-driven feed pump. Admitting steam to the turbine without an open suction valve could result in damage to the pump due to lack of flow and potential cavitation. The other options are incorrect because: A) Securing drains is not necessary for the initial startup, B) Keeping the exhaust valve closed is not required for startup, and C) Keeping the pump casing vent valve closed until flow is established is also not a necessary step for the initial startup.