Final answer:
Ja'far bin Abu Talib responded to King Negus by explaining their belief in the oneness of God and the teachings of Prophet Muhammad. He likely recited verses from the Quran to illustrate the truth of their new faith.
Step-by-step explanation:
Ja'far bin Abu Talib responded to King Negus on why they no longer believed in idols like their forefathers by explaining their belief in the oneness of God and the teachings of Prophet Muhammad. He emphasized that they had found a more enlightened and rational path in Islam. Moreover, Ja'far bin Abu Talib likely recited verses from the Quran to illustrate the truth of their new faith.