Concept:The doubtful day is the day when the new moon of Ramadan is not sighted, making it uncertain whether it is still Sha'aban or the first day of Ramadan.Explanation:This doubtful day falls specifically on the 30th of Sha'aban.On that day, Muslims are not allowed to fast as a precautionary fast for Ramadan.Fasting on that day is prohibited because it is not confirmed that Ramadan has begun.However, a person may still fast if it is his usual voluntary fast or to make up a missed obligatory fast.Answer:D. 30th of Sha'aban