Why 1 year in the muslim calendar so important

The slamic calendar, which is based purely on lunar cycles, was introduced in 638 Christian Era (CE) by Umar ibn Al Khattab, a close companion of the Noble Prophet sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) and the second Caliph of Islam. He did it in an attempt to rationalize the various, at times conflicting, dating systems used during his time.

Umar, may Allah be pleased with him, consulted his advisers on the starting date of the new Muslim chronology. It was finally agreed that the most appropriate reference point for the Islamic calendar was the Hijra. The actual starting date for the calendar was chosen (on the basis of purely lunar years, counting backwards) to be the first day of Muharram, the first month of the year of the Hijra.

Explanation:

after the french-indian war.

i beleive that d is the right answer