In the given question a sentence is given with a blank

Solution

The sentence talks about the potential consequences of pollution on the availability of pure air in the future. The word "vanish" fits best here as it means "disappear" or "cease to exist," which aligns with the context of pure air disappearing due to pollution. Coagulate refers to the process of a substance, especially a liquid, becoming thick or solid, which doesn't fit the meaning needed here. Fulfill means to complete or accomplish something, which does not make sense in the context of pure air disappearing. Gear refers to a mechanism or to adjust or prepare something, which is irrelevant to the given context.