Answer:
A. 1st law.
Step-by-step explanation:
Newton's first law of motion states that, an object or body will continue to be at rest or experience a uniform motion in a straight line at constant speed provided there's no action of an external force. Thus, unless compelled to change its state by an external force, a body will continue to be at rest or experience a uniform motion in a straight line.
The Newton's first law of motion is also known as the law of inertia; which is the tendency of a body to be at rest unless acted upon by an external force.
In this scenario, you are standing in a subway train and the train suddenly stops but your body continues to move forward. This is in accordance with Newton's first law of motion.
Hence, the sudden stoppage of the train generates a force which acts on the body thereby causing the passenger to tip or move forward.