Which of the following is not a leap year?

The concept of leap years is a crucial aspect of our calendar system, aimed at maintaining synchronization with the Earth's orbit around the Sun. A leap year, as you know, has 366 days, including an extra day added to February. This adjustment ensures that our calendar year aligns better with the solar year's actual length.

The rule to determine if a year is a leap year follows these criteria:

If a year is divisible by 4, it is a potential leap year candidate.

However, if a year is divisible by 100, it might not be a leap year, unless...

The year is also divisible by 400, in which case it becomes a leap year.

Now, let's apply this rule to the options:

2000: This year is a leap year. It's divisible by both 4 and 400.

2016: This year is a leap year. It's divisible by 4.

2020: This year is a leap year. It's divisible by 4.

2100: Ah, here's the one that doesn't fit the leap year criteria! While it's divisible by 4, it's also divisible by 100 but not by 400. As a result, 2100 is not a leap year.

In conclusion, out of the given options, the year 2100 is not a leap year. This is due to the fact that it's divisible by 4 and 100, but not by 400, which goes against the criteria we use to determine leap years. Remember, leap years play an essential role in keeping our calendar aligned with the Earth's journey around the Sun, ensuring that our dates and seasons stay relatively synchronized.