Leap-year identification — Which of the following is not a leap year under the Gregorian rules?

Introduction / Context:
Leap year rules: a year is a leap year if divisible by 4; however, century years must be divisible by 400 to qualify.

Given Data / Assumptions:

• Years: 2100, 1200, 1600, 2000.
• Century rule: divisible by 100 → must also be divisible by 400.

Concept / Approach:
Apply the divisibility conditions to each option.

Step-by-Step Solution:
2100: divisible by 100 but not by 400 → not a leap year.1200: divisible by 400 → leap year.1600: divisible by 400 → leap year.2000: divisible by 400 → leap year.

Verification / Alternative check:
Gregorian calendar explicitly treats 1700, 1800, 1900, 2100 as common years; 1600, 2000 as leap years.

Why Other Options Are Wrong:
1200/1600/2000 meet the 400-divisibility requirement, so they are leap years.

Common Pitfalls:
Assuming “divisible by 4” alone is sufficient for century years.

Final Answer:
2100 is not a leap year.