The correct answer is option 2: Newton`s first law. Newton`s first law, also known as the law of inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an external force.

When a stationary bus starts moving forward, our body tends to resist the change in motion due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. Since our body was initially at rest, it wants to stay at rest according to Newton`s first law. But when the bus starts moving, an external force acts on our body, pushing it forward and causing it to accelerate.

As a result of this acceleration, our body tries to remain at rest but the bus`s motion is pushing us forward. This creates a difference between our body`s initial state of motion and the bus`s motion, leading to a jerk or a sudden movement backward.

So, the explanation for why we experience a jerk when a stationary bus starts moving forward is due to Newton`s first law of motion, which highlights the concept of inertia.