Ans: B
$\begin{array}{rcl}
x^2 + mx + n & \equiv & (x+4)(x-m) + 6 \\
x^2 + mx + n & \equiv & x^2 -mx + 4x -4m + 6 \\
x^2 + mx + n & \equiv & x^2 + (4-m)x + (6-4m)
\end{array}$
$\begin{array}{rcl}
x^2 + mx + n & \equiv & (x+4)(x-m) + 6 \\
x^2 + mx + n & \equiv & x^2 -mx + 4x -4m + 6 \\
x^2 + mx + n & \equiv & x^2 + (4-m)x + (6-4m)
\end{array}$
By comparing the coefficients of both sides, we have
$\left\{ \begin{array}{ll}
m = 4-m & \ldots \unicode{x2460} \\
n = 6-4m & \ldots \unicode{x2461}
\end{array} \right.$
From $\unicode{x2460}$, we have
$\begin{array}{rcl}
m & = & 4 – m \\
2m & = & 4 \\
m & = & 2
\end{array}$
Sub. $m=2$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
n & = & 6-4(2) \\
n & = & -2
\end{array}$
