Concept:A follower alerts the Imam during prayer when he makes an unintentional mistake, such as standing instead of sitting.Explanation:When the Imam forgetfully rises at a time he should sit, the followers say "Subhānallah" to draw his attention."Subhānallah" means "Glory be to Allah".It is the proper phrase used to correct the Imam while prayer is in progress.The other options do not serve this purpose in the given situation.Answer:C. Subhānallah