Who is the fifth of the Rightly Guided Caliphs, and is he Omar ibn Abd al-Aziz or Hasan ibn Ali ibn Abi Talib, and when did Omar ibn Abd al-Aziz assume power?

The fifth Rightly Guided Caliph is Al-Hasan ibn Ali, may Allah be pleased with him, due to the saying of the Prophet, peace and blessings be upon him: "The Caliphate among my nation will last thirty years." His caliphate included the six months that completed this period. As for Mu'awiyah and Umar ibn Abd al-Aziz, they were just and rightly guided. Mu'awiyah was the first of the Muslim kings and the best of them, and kingship was intertwined with his rule, but that did not detract from his justice. Umar ibn Abd al-Aziz was called the fifth Caliph because he revived the guidance of the Rightly Guided Caliphs at a time when rulers had strayed from it.