The answer would be C. We know that d is equal to the initial depth of a lake. The two given initial depths are 58 feet and 53 feet, so we know that one of the equations must be either d=58 or d=53. Because there only C has either one of those, d=58, we know that it must be the answer.
To find the other equation, it is just a linear function for the other lake. The y-intercept, or initial value, is 53, so in the equation y=mx+b, it is the b value. The slope, or m value, is 3 feet, so you have y=d=3x+53.