The four Rightly Guided Caliphs (الخلفاء الراشدون) in order are: Abu Bakr al-Siddiq, Umar ibn al-Khattab, Uthman ibn Affan, and Ali ibn Abi Talib. The question asks who is considered the fifth.
عمر بن عبد العزيز (Umar ibn Abd al-Aziz), the Umayyad caliph who ruled from 99 to 101 AH (717-720 CE), is widely regarded in Islamic tradition as the "fifth Rightly Guided Caliph." He earned this title because of his exceptional piety, justice, and commitment to ruling according to the principles established by the first four caliphs. He reversed many of the controversial policies of earlier Umayyad rulers, returned unjustly seized properties, and lived a notably austere life despite being head of a vast empire.
The Prophet Muhammad (peace be upon him) is reported in hadith traditions to have foretold a just ruler from the Umayyad family, which scholars have applied to Umar ibn Abd al-Aziz.
The other options are incorrect: الحسن بن علي held the caliphate briefly before ceding it to Muawiyah, but he is not traditionally given this specific title. أبو عبيدة was a prominent companion but never held the caliphate.
