The correct answer is C. Above 50 kmph.
Isolation is a safety measure that is used to prevent trains from colliding. It is achieved by physically separating the tracks so that trains cannot pass each other. Isolation is required at all interlocked stations, which are stations where trains can pass each other. The speed at which isolation is required varies depending on the country and the type of track. In the United States, isolation is required at all interlocked stations where the speed is above 50 kmph.
Option A is incorrect because isolation is required at all interlocked stations, not just those with a speed of 50 kmph. Option B is incorrect because isolation is required at all interlocked stations, not just those with a speed of 75 kmph. Option D is incorrect because isolation is required at all interlocked stations, not just those with a speed of 50 kmph or 75 kmph.