There are two components of acceleration felt by the mass of a stationary pendulum inside the truck.

Vertical acceleration of $9.81 m/s^2$ due to gravity & inertial acceleration of '3b' opposite to direction of motion of the trunk.

The pendulum takes a mean position such that $T \sin \alpha= ma\;\& T \cos \alpha=mg$ where T is the tension in the string & is the angle string holding mass 'm' of the pendulum made to the vertical.

Thus $\tan \alpha=\large\frac{3b}{g}$ or $\alpha$ in radians $=\large\frac{3b}{g}$ for small angles.

Hence b is the correct answer