Concept:Voluntary fasting is an act of worship encouraged by the Prophet Muhammad (peace be upon him) on certain days.Explanation:The Prophet recommended fasting on Mondays and Thursdays.He said that deeds are presented to Allah on Mondays and Thursdays.Among the given options, only Mondays is a day on which voluntary fasting is specifically encouraged.Fasting on the two 'Id days is forbidden, and fasting only on the day before Ramadan is not a confirmed encouragement.Answer:D. Mondays