Final answer:
The scientific purpose of an airbag is to extend the impulse of impact in a crash, lowering the average force exerted on passengers by increasing the time over which this force acts.
Step-by-step explanation:
The purpose of a car's airbag during a crash is to extend the impulse of impact, which is the product of the net force and the time over which the force acts. This extension of time leads to a decrease in the average force experienced by the occupants. The airbag achieves this by cushioning the impact against the steering wheel or dashboard, allowing for a longer collision time, which means lower force on the car and its occupants. The momentum change of the individual involved in the collision remains constant, whether an airbag is deployed or not, but with the presence of an airbag, the force exerted to bring the occupant to a stop is considerably lessened.
Additionally, cars with components that crumple also contribute to safety as they allow for a longer collision time as well, which reduces the force exerted during an impact.
To directly answer the question, the scientifically correct purpose of a car's airbag during a crash is primarily b) Extends the impulse of impact so the average force decreases.