The sentence that contains an adverb clause is "The Lehigh Valley, in eastern Pennsylvania, has seven covered bridges."
Explanation:
An adverb clause works as a regular adverb and they belong to the same categories, the sentence D has an adverb clause with an adverbial of place, this clauses commonly start with a preposition which in this sentence is in, this tell the place where the things happen or where the things are, then this one specifically describes where the Lehigh Valley is.
