The correct answer is $\boxed{\text{B. }8 \text{ sec}}$.
The stone’s initial velocity is $v_0 = 49 \text{ m/s}$ and its acceleration is $a = -9.8 \text{ m/s}^2$ (negative because it is directed downwards). The time it takes to reach the top of its trajectory is given by
$$t = \frac{v_0}{a} = \frac{49 \text{ m/s}}{-9.8 \text{ m/s}^2} = 5 \text{ s}$$
The stone’s velocity at the top of its trajectory is zero, so its time to fall back down is also $5 \text{ s}$. Therefore, the total time it takes to return to the ground is $5 + 5 = \boxed{8 \text{ sec}}$.
Option A is incorrect because it is the time it takes the stone to reach the top of its trajectory. Option C is incorrect because it is twice the time it takes the stone to reach the top of its trajectory. Option D is incorrect because it is four times the time it takes the stone to reach the top of its trajectory.