Adverb phrase in "Parvo is a common disease in dogs caused by a virus in the air."

The correct answer here is "in the air". This phrase acts as an adverb of place as it describes where the virus is located. These phrases can modify a verb, adverb and adjectives and tells us more information about them like in this example "where" something is. They can also tell us about "how" or "when" or "why".