What is the frequency of leap years in the Gregorian calendar?

In the Gregorian calendar, leap years occur to help align the calendar year with the Earth's orbit around the Sun, which takes approximately 365.24 days. The basic rule is that a year is a leap year if it is divisible by 4. However, the Gregorian calendar introduces additional criteria to account for discrepancies that arise over centuries.

The statement that leap years occur every 4 years, except for century years that are not divisible by 400, is essential because it prevents excessive accumulation of leap years over long periods. For instance, while the year 2000 was a leap year (as it is divisible by 400), the year 1900 was not (as it is not divisible by 400). This special rule ensures that the average length of the calendar year stays closer to the true solar year.

Thus, the correct understanding is that leap years occur every 4 years, with the important caveat for century years that are not divisible by 400. This makes option D the most accurate and comprehensive answer regarding leap years in the Gregorian calendar.