Which of the following is equal to $9^{4}$ ?
(A) $3^{2}$
(B) $3^{4}$
(C) $3^{6}$
(D) $3^{8}$
(E) $3^{10}$
Since $9=3 \times 3$, then $9^{4}=(3 \times 3)^{4}=3^{4} \times 3^{4}=3^{8}$.

Alternatively, we can note that

$$
9^{4}=9 \times 9 \times 9 \times 9=(3 \times 3) \times(3 \times 3) \times(3 \times 3) \times(3 \times 3)=3^{8}
$$

ANSWER: (D)