Final answer:
The birth of the Islamic leader whom Muslims base their calendar system on is a) Prophet Muhammad. In Shia Islam, the legitimate caliphs and Imams must be descended from Ali, who was the cousin and son-in-law of Muhammad. Islam is a monotheistic religion that recognizes the same God in common with Judaism and Christianity.
Step-by-step explanation:
Muslims use a calendar system that is based on the hijra, the migration of the Islamic prophet Muhammad from Mecca to Medina. The correct answer to the question regarding the birth of their leader, who Muslims base their calendar on, is a) Prophet Muhammad. Just like Christian history is often marked from the birth of Jesus, Islamic history is marked from the migration of Muhammad, an event that occurred in 622 CE, which is designated as year 1 in the Islamic calendar.
In Shia Islam, legitimate caliphs had to be descended from Ali, who was the cousin and son-in-law of Muhammad. Ali is considered the first legitimate Imam and spiritual leader of the Shia Muslims. It's important to note that while Sunni Muslims constitute the majority of Muslims around the world, the Shia Muslims hold the belief that Ali and his descendants are the rightful successors to Muhammad. To clarify the second part regarding Muslim beliefs, Muhammad is indeed perceived as the final prophet in a long line of monotheistic prophets and Islam is regarded as a monotheistic faith, recognizing the same Abrahamic God worshiped by Jewish people and Christians.
The founding of Islam and its spread, including the role of caliphs succeeding Muhammad, played a pivotal role in historical events. Ali's leadership and his descendants have been crucial to the development of Shia Islam. Additionally, the connection between Islamic teachings and the succession of prophets is foundational to understanding Muslim beliefs.