Final answer:
The non-vertical hoist line introduces a lateral force to the crane, potentially causing instability due to bending moments.
Step-by-step explanation:
When the hoist line is not vertical, the type of dynamic force being introduced to the crane is B. Lateral force. A lateral force acts perpendicular to the vertical axis of a structure, in this case, the crane. This force can cause bending moments within the crane structure, potentially leading to instability. The dynamic nature of this force arises because the off-vertical alignment of the hoist line can lead to the crane swinging and movement of the load, which dynamically changes the forces involved.