The rate of change is the slope.
That equation is (practically) in the form of y=mx+b because T(d) is very similar to y.
T(d) = 39.27d+36.77
y = mx+b
In this case, d would equal x. T(d) would equal y. 36.77 would equal b.
m is the slope, or the rate of change.
The rate of change is the variable, d, or the answer choice, a: the number of drinks purchased.