Answer:
B. Around forts; modifies were built 
Step-by-step explanation:
An adverb phrase is a kind of prepositional phrase that functions as an adverb. Therefore, it answers the questions Where? When? How? or Why?  and modifies a verb. 
In the second sentence, we have an adverbial phrase:
around forts
It modifies the verb 'were built'. It is answering the question "Where?"