Concept:Āshura is a recommended (supererogatory) fast observed in the Islamic month of al-Muharram.Explanation:The Prophet Muhammad (S.A.W.) encouraged Muslims to fast on Āshura.Āshura falls on the 10th day of al-Muharram.Therefore, the correct option is the 10th of al-Muharram.Answer:C. 10th of al-Muharram