A student obtains a sharp image of the distant window (W) of the school laboratory on the screen (S) using the given concave mirror (M) to determine its focal length. Which of the following distances should he measure to get the focal length of the mirror?

The concave mirror forms the image of the distant object at its focus. Thus from the figure (which is implied but not explicitly shown in the problem description), MW is the object distance while MS is the image distance which is equal to the focal length of the mirror. Thus to determine the focal length of the mirror, the student needs to measure MS.