The term that best describes a covered prayer hall is letter B. Mihrab. Mihrabs should not be confused with minbar, which is a raised platform where the Imam stands. The term Haram is forbidden or proscribed by the Islamic law and faith. MIhrab is a part of a mosque, it is also considered as a semicircular niche in the walls of the mosque that indicates qibla. That is the direction of Kaab in Mecca, and hence where the Muslim should face during praying.
