Answer: Option (a) is the correct answer.
Step-by-step explanation:
It is known that molecules of a gas are held by weak Vander waal forces. As a result, they are able to move freely from one place to another. Hence, they are able to spread or diffuse readily into the atmosphere.
Also, gases do not have a definite volume. Whereas molecules of a liquid are held together by slightly strong forces as compared to gases. As a result, liquids do not have a fixed shape and they occupy the volume of a container in which they are kept.
Therefore, we can conclude that the sentence gases do not have a definite volume, explains that when a perfume bottle is opened, some liquid changes to gas and the fragrance spreads around the room.