The mean of the squares of the first n natural numbers is

A

$$\frac{1}{6}(2n^2 + 3n + 1)$$

B

$$\frac{1}{3}(2n^2 + 3n + 1)$$

C

$$\frac{1}{6n}(2n^2 + 3n + 1)$$

D

$$\frac{1}{6} n (n + 1)(2n + 1)$$