Which of the following is not a leap year?

Every year divisible by 4 is a leap year, if it is not a century. Every 4th century is a leap year, but no other century is a leap year. 2000,1964 and 2020 comes in the category of 4th century.

1900 is not divisible by 4 and 400

Hence, 1900 is not a leap year.