The real word meaning of "slope" refers to the steepness or incline of a surface, usually represented as the ratio of the vertical rise to the horizontal run. It is typically used in mathematical and geographical contexts to describe the angle of a line or hill.
None of the options provided (A, B, or C) accurately define "slope" in its typical usage. If you're looking for the proper definition, it would be related to incline rather than services offered by a plumber.