During which Caliph's reign was the Islamic calendar (Hijri calendar) officially establish

A) Abu Bakr As-Siddiq

B) Umar ibn Al-Khattab

C) Uthman ibn Affan

D) Ali ibn Abi Talib

Explanation

Caliph Umar (RA) officially introduced the Hijri calendar, with the Hijra (migration to Madinah) as its starting point.