Explanation:
f(n) = 3(2)^n = 3 * 2^n.
We can see here that 3 is a constant term.
When n = 0, f(0) = 3 * 2^0 = 3.
Hence, on the 1st day 3 leaves fell. (A)
When n = 1, f(1) = 3 * 2^1 = 6.
When n = 2, f(2) = 3 * 2^2 = 12.
Notice that the number of leaves that fell keep doubling.
Therefore the number of leaves that fell on a given day is 2 times the number of leaves that fell on the previous day. (D)