Inertia of Rest
When a car is stationary, the passengers inside the car are also at rest. According to Newton's First Law of Motion, an object at rest will stay at rest unless acted upon by an external force. In this case, the external force is the car suddenly starting to move. When the car accelerates, the passengers inside the car tend to remain at rest due to inertia.

Jerk
Jerk is the rate of change of acceleration with respect to time. In other words, it is the sudden change in acceleration. When a car suddenly starts moving, it experiences a change in acceleration from zero to a positive value. This sudden change in acceleration causes a jerk.

Explanation
When the car starts moving, the wheels exert a force on the ground in the backward direction. As a reaction to this force, the ground exerts an equal and opposite force on the car in the forward direction. According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction.

This forward force applied on the car causes it to accelerate. However, due to the inertia of rest, the passengers inside the car tend to remain at rest. As a result, the passengers experience a backward force acting on them, which is equal in magnitude and opposite in direction to the force applied on the car.

This sudden backward force on the passengers causes a jerk, which is felt as a sudden jolt or push in the opposite direction of the car's acceleration. The passengers' bodies want to stay at rest while the car accelerates forward, resulting in this uncomfortable sensation.

Conclusion
The jerk experienced by passengers sitting in a stationary car when the car suddenly starts moving is due to the inertia of rest. The passengers tend to remain at rest due to their inertia, and when the car accelerates forward, they experience a sudden backward force, causing the sensation of a jerk.