Why is air a mixture but water a compound?

Water is considered a compound because it is formed by the combination of two elements, h and o which are chemically bonded to form a molecule of water.
however in air the elements comprising it, nitrogen, oxygen etc are not chemically bonded but rather exist in separate physical states, air also contains other compounds such as co2 and other green house gases (ch4) all of this is mixed together but can be separated by physical means which is why it is called a mixture.

The answer is b because a,c,and d are opinons

Bthats what im doing to and i figured it out it is b