The correct answer is 'c) Pope Gregory XIII'.

The Gregorian calendar was established by Pope Gregory XIII in 1582. It was named after him to honor his contribution to the reform of the Julian calendar, which had been in use for over 1600 years.

Pope Gregory XIII took the initiative to reform the calendar in order to correct the inaccuracies that had accumulated over time. The Julian calendar, established by Julius Caesar in 45 BCE, had a slight miscalculation of the solar year, causing the calendar to drift out of synchronization with the seasons. This discrepancy had reached about 10 days by the 16th century, causing significant confusion and inconvenience in various aspects of life, particularly in relation to the celebration of Easter.

To address this issue, Pope Gregory XIII commissioned a group of astronomers and mathematicians to devise a new calendar system that would accurately reflect the solar year. This group, known as the "Commission for the Calendar," included renowned astronomers such as Christopher Clavius.

The reform proposed by the Commission for the Calendar involved several adjustments to the existing Julian calendar. The most significant change was the introduction of the leap year rule, which modified the leap year pattern to align the calendar with the solar year more accurately. Under the new rule, years divisible by 100 would not be leap years unless they were also divisible by 400. This adjustment ensured that the average length of the calendar year closely matched the solar year.

To implement the new calendar, Pope Gregory XIII issued a papal bull titled "Inter gravissimas" on February 24, 1582. The bull decreed that the day following October 4, 1582, would be October 15, thus skipping ten days to bring the calendar back in line with the solar year. This adjustment was accepted and adopted by several Catholic countries, including Spain, Portugal, Italy, and Poland.

Over time, the Gregorian calendar gained widespread acceptance and was eventually adopted by most countries around the world. It is now the most widely used civil calendar globally and serves as the basis for international standard timekeeping.

In conclusion, Pope Gregory XIII established the Gregorian calendar in 1582 as a reform of the Julian calendar to correct inaccuracies and bring the calendar back in sync with the solar year. His efforts to improve the accuracy of timekeeping have had a lasting impact, and the Gregorian calendar remains in use to this day.