Concept:The "doubtful day" is the day when it is uncertain whether Ramadan has begun, which falls at the end of Sha’aban.Explanation:The doubtful day is the day on which the new moon of Ramadan may or may not have been sighted.This day occurs on the 30th of Sha’aban when there is no clear confirmation of the sighting.Fasting on this doubtful day is prohibited because it is not confirmed as Ramadan.Answer:D. 30th of Sha’aban