Whatever unit is given on the top (miles), you will write it on the bottom of the second fraction. Since we are solving for length in feet, we know that the numerator will be in feet. The only answer choice that has this option is 5280 ft/ 1 mi. You can check this math by taking 0.35 of 5280 feet, or 1 mile = 1848 ft.
The independent variable is M, Miles. This is because you can walk however miles without any effect. But calories depends on how much you walked. Make sense?