Ans: C
$\begin{array}{rcl}
\text{P(drawing a red ball)} & = & \dfrac{1}{4} \\
\dfrac{12}{n+12} & = & \dfrac{1}{4} \\
48 & = & n + 12 \\
n & = & 36
\end{array}$
$\begin{array}{rcl}
\text{P(drawing a red ball)} & = & \dfrac{1}{4} \\
\dfrac{12}{n+12} & = & \dfrac{1}{4} \\
48 & = & n + 12 \\
n & = & 36
\end{array}$
