The correct answer is: None of these.
The amount of vacuum level required in a high-speed train locomotive is not a fixed value. It depends on a number of factors, including the weight of the train, the speed of the train, and the type of track. In general, however, the vacuum level required in a high-speed train locomotive is between 50 and 60 cm of mercury.
Option A: 50 cm is a possible value for the vacuum level required in a high-speed train locomotive, but it is not the only possible value.
Option B: 55 cm is a possible value for the vacuum level required in a high-speed train locomotive, but it is not the only possible value.
Option C: 56 cm is a possible value for the vacuum level required in a high-speed train locomotive, but it is not the only possible value.
Option D: None of these is the correct answer.