Answer
b) is correct.
Work Step by Step
As the total mechanical energy is conserved, the total mechanical energy when the roller coaster reaches the top is the same as the initial mechanical energy: $E_0=E_f$
Since $E=\frac{1}{2}mv^2+mgh$, as the roller coaster goes back to its original position with similar height $h$, it will also go back to its initial speed $v=6m/s$
b) is correct.
