We use the formula

$\begin{align*}E^2 = m^2 c^4 + p^2 c^2\end{align*}$
We are given that $E = 10 \mbox{ GeV}$ and $p = 8 \mbox{ GeV/c}$ , therefore,
$\begin{align*}m = \sqrt{E^2 - p^2 c^2}/ c^2 = \boxed{6 \;\mathrm{GeV}\mathrm{/}\mathrm{c}^2}\end{align*}$
Therefore, answer (D) is correct.

Solution to 2001 Problem 79