9. D.

Let the total number of marbles Macy has be $x$, and the number of yellow marbles be $y$. Then $y=\left(1-\frac{1}{3}-\frac{1}{4}\right) x-6=\frac{5}{12} x-6$.
Note that, $y$ is a monotonically increasing linear function of $x$, and $x, y \in \mathbf{Z}_{+}$.
Therefore, $y_{\min }=24 \times \frac{5}{12}-6=4$.
9. Masi has marbles in blue, red, green, and yellow. One third of the marbles are blue, one quarter are red, and there are 6 green marbles. How many yellow marbles does Masi have at the least?
(A) 1
(B) 2
(C) 3
(D) 4
(E) 5