The underlined adjective clause "that was written" is modifying the noun poem. In this sentence, the adjective is describing or adding further information about the poem. Option C is correct.
An adjective clause is a form of dependent clause which purpose is to describe a noun in a sentence. It works as an adjective even though it is made up of a group of words instead of just one word. In the case of an adjective clause, all the words are intended to modify or describe the noun or pronoun.
