Jose gets up from his seat on the bus to move closer to the front. Just as he begins to walk forward, the bus stops at a light. What is the best explanation of what happens to Jose?

According to first law of motion, a body continues to be in state of rest or motion unless an unbalanced force acts on it.

When the bus stops, Jose feels a sudden forward motion. This is due to the inertia of motion. In the bus, Jose has same speed as the bus while sitting. When she gets and starts moving, the bus stops at the light. But due to the inertia of motion, Jose feels a sudden forward motion before she finally stops.