Question

if youre in a car that stops suddenly, you feel thrown forward. why does this happen? the cars brakes create a forward force on you. your mass suddenly increases, pushing you forward. the car stops, but your bodys inertia makes it continue moving forward until a force (like the seatbelt) stops you. the air inside the car pushes you forward.

Explanation:

According to Newton's first law of motion (the law of inertia), an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an external force. In this case, when the car stops suddenly, the car experiences a force (from the brakes) to stop its motion. However, your body, which was in motion along with the car before braking, has inertia (a property of matter to resist changes in its state of motion). So, your body tends to continue moving forward until a force (like the seat - belt) acts on it to stop its forward motion.
The car's brakes create a backward force on the car (not a forward force on you), so the first option is wrong. Mass is a property of an object and does not suddenly increase in this situation, so the second option is wrong. The air inside the car is not the main reason for you feeling thrown forward, so the fourth option is wrong.

Answer:

The car stops, but your body's inertia makes it continue moving forward until a force (like the seatbelt) stops you.