Concept:An emissary is a messenger sent by the Prophet to deliver an important message or demand to another group.Explanation:The Jewish tribe of Banu Nadir was part of the community of Medina but later planned to act treacherously against the Prophet.The Prophet received information about their plot and needed someone to deliver his message to them.He therefore sent Ali b. Abi-Talib to them as his emissary.Ali conveyed the Prophet’s message warning them against breaking their covenant and reminding them of the consequences of their attitude.The Banu Nadir refused to accept the message, and this eventually led to their expulsion from Medina.Answer:B. Ali b. abi-Talib.