Concept:Voluntary fasting is a recommended act of worship in Islam, performed on certain meritorious days.Explanation:The Prophet Muhammad (PBUH) encouraged Muslims to fast voluntarily on Mondays and Thursdays.He explained that actions are presented to Allah on these two days, so he loved that his deeds be shown while he was fasting.Among the listed options, Mondays are specifically recommended for voluntary fasting in authentic Hadiths.Fasting on Id days is forbidden, and fasting on the day before Ramadan without a prior habit is not generally encouraged.Answer:D. Mondays