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.

that’s the kind of bug henry was; and if we’d a had him along ’stead of our kings, he’d a fooled that town a heap worse than ourn done.”

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."