Answer
The force vector $b$ in the diagram shows the correct location and direction of the force.
Work Step by Step
The torque is $\tau = r\times F$, where $r$ is the smallest distance between the force vector and the axis, and $F$ is the perpendicular component of the force vector.
To push the box over with the minimum force, we need to maximize the torque. We can maximize the torque by maximizing $r$. Clearly, the force $b$ in the diagram maximizes $r$ which in turn maximizes the torque.
The force vector $b$ in the diagram shows the correct location and direction of the force.
