Final answer:
The depth of the lake, 50 feet below the surface, is represented by the number -50 feet, indicating it is measured in the negative direction from the surface.
Step-by-step explanation:
The depth of the lake can be represented by a negative number when measuring from the surface down to the bottom.
In this case, since the bottom of the lake is 50 feet below the surface, the number that can represent the depth of the lake is -50 feet. Negative numbers are used in this context because depth is measured in a direction opposite to the positive reference direction, which is typically upwards or away from the center of the Earth.