Answer:
Ayudó.
Explanation:
Helping someone is most commonly a one time occurance, meaning that you will need to use the preterite form.
When conjugating, 'usted' is conjugating the exact same as 'ella' or 'él' meaning that for 'ar' verbs, you will need to replace the ending with an 'ó'.
Therefore, ayudar --> ayudó.