Ans: C
$\begin{array}{rcl}
f(4)-f(3) & = & 21 \\
(4)^2 +k(4) + 7 -[(3)^2+k(3)+7] & = & 21 \\
16 + 4k + 7 -9 -3k-7 & = & 21 \\
k & = & 14
\end{array}$
$\begin{array}{rcl}
f(4)-f(3) & = & 21 \\
(4)^2 +k(4) + 7 -[(3)^2+k(3)+7] & = & 21 \\
16 + 4k + 7 -9 -3k-7 & = & 21 \\
k & = & 14
\end{array}$
