The correct answer is:
A. Hijrah
Explanation:
Muhammad's vision of Archangel Gabriel let him with the prophesy that there was only one God, and he started to gain followers so Mecca citizens started to lose money, and planned on capturing Muhammad, but Muhammad escaped with his followers and settled in Medina. This migration, known as Hijrah, or Hegira is known as the beginning of the Islamic era, and is marked as the year 1 of the Muslim calendar.