Ans: C
$\begin{array}{cl}
& f(c) + f(-c) \\
= & [c^3 + c(c)^2 + c] + [(-c)^3 +c(-c)^2 + c] \\
= & c^3 + c^3 + c – c^3 + c^3 + c \\
= & 2c^3 + 2c
\end{array}$
$\begin{array}{cl}
& f(c) + f(-c) \\
= & [c^3 + c(c)^2 + c] + [(-c)^3 +c(-c)^2 + c] \\
= & c^3 + c^3 + c – c^3 + c^3 + c \\
= & 2c^3 + 2c
\end{array}$
