Which of these isn't a leap year?
1990
Years which are divisible by 4 are leap years. For example, 1904, 1908,...,2000, 2004,...are leap years. The exception is: The century years (i.e., 1800, 1900, etc) even though they're divisible by 4, are not leap years. But the years divisible by 400, will be leap year. e.g. the year 2000 was a leap year and the year 2400 will also be a leap year.
∴ 1900, a century year isnt a leap year(even though divisible by 4).