The Christian calendar, which dates time from the birth of Jesus, was formulated by:. a. A sixth-century abbot.
b. Constantine.
c. Augustine.
d. Justinian.

Option a (a sixth-century abbot) is the appropriate choice.

Explanation:

The Christian calendar seems to be a periodic schedule that memorializes some specific holidays throughout events to help us understand the significant works of God throughout the process of regeneration. Some religions give to every day during the year with a special interpretation of Scripture as well as occurrence or individual in the church's history.The above calendar becomes conceived by that of the abbot of the sixth century, who laid the cornerstone of Christian monasticism throughout continental Europe.

Such decisions are taken aren't relevant to the situation. It is the right answer to the above.