Answer
c) is correct.
Work Step by Step
The principle of energy conservation states that: $$E_f=E_0$$ $$\frac{1}{2}mv_f^2+mgh_f=\frac{1}{2}mv_0^2+mgh_0$$ $$\frac{1}{2}m(v_f^2-v_0^2)=mg(h_0-h_f)$$
So according to this principle, as any vehicle moves up the hill $(h_0-h_f\lt0)$, it also follows that $(v_f^2-v_0^2)\lt0$, or the vehicle's speed must decrease.
Therefore, the sports car, maintaining its constant speed by its engine, does not obey the principle of conservation of mechanical energy.
The truck, on the other hand, by turning off its engine, has eliminated the nonconservative force. Only conservative forces are left, so the principle of conservation of mechanical energy is obeyed.
