An exponential or geometric function can be expressed as a power of t, where t is time.
This means that if you can fit all three values into the formula
S = S0 * (1+r)^t
for a constant r, and t=1, 2, 3 (or 0, 1, 2 for simplicity), then it's exponential.
You can see right away that the first and second sets of numbers are not exponential. These are linear, because each month is a fixed value greater than the previous one.
If you look at the formula above, you can see that each successive time interval's growth can be calculated by multiplying a fixed value to the previous intervals. For example, the second month is given by:
S(1) = S0 * (1+r)
S(2) = S0 * (1+r)^2 = S0 * (1+r) * (1+r) = S(1) * (1+r)
Since each month's sales is 102% the previous month's in the fourth set, this is the one you want.
