A = P (1 + r / 100)ᵗ
81P = P (1 + r / 100)⁴
Canceling P from both sides:
81 = (1 + r / 100)⁴
Taking the fourth root:
∜81 = 1 + r / 100
3 = 1 + r / 100
Solving for r:
r / 100 = 2
r = 200%
Thus, the correct answer is 200% (Option D).