In the anthology, i read a classic poem underline starts that was written underline ends over 500 years ago. which word does the underlined adjective clause modify?
a)anthology
b)classic
c)poem
d)years

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.

in june, we will have been married for eighteen months. (i am assuming this is what you want? the sentence, as you have presented it, is grammatically incorrect--   you want it corrected, yes? )

remember, the goal of an adjective clause is to add more information to a noun or a pronoun. as you can see from the examples above, you can add information by including a longer adjective clause or tighten up a sentence by turning the adjective clause into an adjective phrase.