What is the function of a life vest during a flight?

The function of a life vest during a flight is specifically designed for use in emergency situations, particularly if an aircraft has to make an emergency landing on water. Life vests provide buoyancy, helping passengers stay afloat and increasing their chances of survival in the event of a water landing. These vests are equipped with features such as inflation mechanisms and reflective surfaces to enhance visibility.

The other choices don't align with the primary purpose of a life vest. For instance, while entertainment during a flight is important, it is not related to the function of a life vest. Wearing the vest while boarding the aircraft is not standard practice, as the vests are not intended to be used or put on until an emergency occurs. Lastly, maintaining altitude pertains to the operation of the aircraft itself, which is unrelated to passenger safety equipment like life vests.