Which of the following designations signifies a mile marker on the railroad?

The designation that signifies a mile marker on the railroad is "MP," which stands for "Mile Post." It is a widely recognized term used within the railroad industry to indicate the distance in miles from a specific reference point, often the beginning of a track segment. Mile posts are essential for navigation and operational planning, helping crews identify their location on the railroad and ensuring accurate reporting and communication regarding distances traveled.

The other designations do not represent the standard terminology for mile markers in railroading. "MPM" and "MEM" do not have common usage related to mileage tracking, while "MLP" does not pertain to a mile marker either. Thus, "MP" is the correct and standard designation used across railroads to mark mile posts.