答案：C
$\begin{array}{rcl}
(x + h)(x+6) & \equiv & (x+4)^2 + k \\
x^2 + (h + 6)x + 6h & \equiv & x^2 + 8x + 16 + k \\
\end{array}$
$\begin{array}{rcl}
(x + h)(x+6) & \equiv & (x+4)^2 + k \\
x^2 + (h + 6)x + 6h & \equiv & x^2 + 8x + 16 + k \\
\end{array}$
透過比較兩方的係數，可得
$\left\{\begin{array}{ll}
h + 6 = 8 & \ldots \unicode{x2460} \\
6h = 16 + k & \ldots \unicode{x2461}
\end{array}\right.$
從 $\unicode{x2460}$，可得 $h = 2$。
把 $h = 2$ 代入 $\unicode{x2461}$，可得
$\begin{array}{rcl}
6(2) & = & 16 + k \\
k & = & -4
\end{array}$
