Dua of the one fasting until he breaks his fast.
The Messenger of Allah (salAllahu alayhi wasalam) said; Three supplications will not be rejected (by Allah (Subhanahu wa Ta’ala)), the supplication of the parent for his child, the supplication of the one who is fasting, and the supplication of the traveler. [al-Bayhaqi, at-Tirmidhi - Sahih]