The correct answer is option 'B', Gregory XIII.

Introduction to the Gregorian Calendar:
The Gregorian calendar is the most widely used calendar system in the world today. It was introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar. The Julian calendar, implemented by Julius Caesar in 45 BCE, was based on a solar year of 365.25 days, which was slightly longer than the actual solar year of 365.2425 days.

Reasons for the Introduction of the Gregorian Calendar:
There were a few reasons behind the introduction of the Gregorian calendar:

1. Correcting the Julian calendar's inaccuracy: The Julian calendar had a discrepancy of about 11 minutes and 14 seconds per year, which led to a gradual shift of the calendar year away from the solar year. Over time, this misalignment caused the calendar dates to drift away from the actual equinoxes and solstices.

2. Aligning Easter with the spring equinox: The date of Easter is calculated based on the vernal equinox, which was shifting earlier in the Julian calendar due to its inaccuracy. The Gregorian calendar aimed to restore the alignment of Easter with the spring equinox.

3. Addressing religious and practical concerns: The introduction of the Gregorian calendar also aimed to address various religious and practical concerns. It sought to standardize the date of the Christian holiday of Easter, provide a more accurate reckoning of the passage of time, and facilitate administrative and commercial activities across different nations.

Key Features of the Gregorian Calendar:
The Gregorian calendar introduced several key features to address the issues of the Julian calendar:

1. Leap year adjustment: The Gregorian calendar retained the concept of leap years but made adjustments to reduce the accumulated discrepancy. It stipulated that the years divisible by 100 would not be leap years, except for those divisible by 400.

2. Dropping days during the transition: To align the calendar with the solar year, Pope Gregory XIII ordered the removal of ten days from the calendar. The day following October 4, 1582, was declared as October 15, 1582.

3. Modification of the calculation of Easter: The Gregorian calendar introduced a new method for determining the date of Easter, which took into account the corrected vernal equinox.

Significance and Adoption of the Gregorian Calendar:
Initially, the Gregorian calendar was adopted by Catholic countries in Europe, such as Italy, Spain, Portugal, and Poland. However, it gradually gained acceptance in Protestant and Orthodox countries as well.

The adoption of the Gregorian calendar was not immediate worldwide. Different countries adopted it at different times, leading to a disparity in the calendar systems used across regions for several centuries.

Conclusion:
In conclusion, Pope Gregory XIII introduced the Gregorian calendar in 1582 as a reform of the Julian calendar. The reform aimed to correct the inaccuracies of the Julian calendar, align Easter with the spring equinox, and address religious and practical concerns. The Gregorian calendar is now the most widely used calendar system globally, with its adoption varying across countries and regions.