The slope of a function
f(x) = mx + b
is m, i.e., the constant multiplying the variable x.
Thus, for the given function
S = 400 + 0.15c
c is the variable (sales). So, the slope is 0.15.
Also, for each dollar in sales, the initial value of 400 increases by 0.15 * 1 = 0.15 dollars.
Therefore, the correct answer is the first option.