Which revision of this sentence creates the best parallel structure?

to the cause you can write a check, collect donations, or signing the petition.

replace the word collect with the word collecting.

replace the word signing with the words to sign.

replace the word write with the word writing.

replace the word signing with the word sign.

The revision of the sentence that creates the best parallel structure is D. replace the word signing with the word sign.
A parallel structure means that the same form of the word or phrase is used in both sentences or parts of the same sentence. So here, you can see that the words are help, write, and collect, so in order for a sentence to have a parallel structure, the final verb should be sign, without the word to in front of it, or -ing at the end of that word.