You are standing in a moving bus, facing forward, when you suddenly slide forward as the bus comes to an immediate stop. What force caused you to slide forward?

No force caused the person to move to slide forward, instead, inertia is behind this slide.

Inertia is the tendency of the body to remain in a particular state i. e. either moving or at rest unless an external force is applied to it.

Initially, the bus is moving forward so the upper body remains in the inertia of bus while the lower body which is in contact with the bus immediately stops as soon as brakes are applied and this difference in inertia causing the person to slide forward.