The correct answer is D. 1.0.
The impact factor is a factor that is used to account for the dynamic loads that are imposed on a structure by moving loads, such as trains. The impact factor is typically taken as 1.0 for railway bridges carrying a single track. This is because the dynamic loads imposed by trains are relatively small compared to the static loads.
Option A is incorrect because the impact factor is never taken as 0. This is because the dynamic loads imposed by trains will always cause some additional stress on the structure.
Option B is incorrect because the impact factor is typically taken as 1.0 for railway bridges carrying a single track.
Option C is incorrect because the impact factor is not typically taken as a value between 0.5 and 1.0 for railway bridges carrying a single track.