Answer:
The adjective clause in the sentence is "which inspired Francis Scott Key".
Step-by-step explanation:
Adjective clauses are those groups of words or phrases that modify the nature of the noun. They are dependent clauses describing the noun to modify them. They usually have a subject and a verb in it.
From the above provided sentence,
The tattered flag, which inspired Francis Scott Key, was on display at the museum
"the tattered flag" is the noun. The dependent clause that works in support of the noun is "which inspired Francis Scott Key". This clause has a subject "francis Scott Key" as well as a verb "inspired'. Thus, the adjective clause in the sentence is "which inspired Francis Scott Key".