The correct answer is C) insufficient circulating water flow through the condenser. A turbo-generator back pressure trip is designed to protect the turbine from damage due to excessive exhaust pressure. Insufficient circulating water flow through the condenser can cause the exhaust pressure to rise, triggering the back pressure trip. This is because reduced condenser cooling leads to higher exhaust pressure, which could potentially damage the turbine blades. The other options are incorrect because they do not directly cause the back pressure trip to actuate. An excessive pressure drop through the turbine (A) or a steam inlet valve being partially open (B) would not necessarily result in high exhaust pressure. Similarly, excessively low exhaust pressure (D) would not trigger the back pressure trip, as the purpose of the trip is to protect against high exhaust pressure.