Answer:
To help the cause, you can write a check, collect donations, or sign the petition.
Step-by-step explanation:
Parallel structure is the use of the similar or same structure of words in a sentence. It is the use of the same grammatical form within a sentence.
In the given sentence, the parallel structure should be in the verbs "write", "collect" and "sign". And since the last verb "sign" is used in the continuous form while the first two are in their present tense form, it will also be changed according to the tense of the first two.
Thus, the final parallel structure of the given sentence will be
"To help the cause, you can write a check, collect donations, or sign the petition."