Which sentence contains an adverb clause?
a.
the lehigh valley, in eastern pennsylvania, has seven covered bridges.
b.
covered bridges remind us of days gone by.
c.
if you visit pennsylvania, take the self-guided, covered-bridge tour.
d.
only 1,600 covered bridges exist in the world.

C). If you visit Pennsylvania, take the self-guided, covered-bridge tour.

Explanation:

An adverb clause consists of a subject and a verb and begins with subordinating conjunction like if, although, because, etc. It functions as an adverb in a sentence and modifies a verb, adjective or adverb in the sentence that adds significant description and information to a sentence. According to this question, the sentence that employs an adverbial clause "If you visit Pennsylvania" in it, so, it is option C as it meets and justifies all the descriptions and claims as mentioned above. It begins with 'if'(subordinating conjunction), subject(You) and verb(visit) and adding a description to the sentence. Thus, option C is the correct answer.