The leap year occurs after every

Answer: (c) 4 years

A leap year comes every 4 years (except century years not divisible by 400).

The idea

The seasonal year is slightly longer than 365 days. A calendar with only 365-day years would slowly drift against the seasons. Leap days correct most of that drift, with century exceptions preventing too many extra days.

Seasonal year is about 365.2422 days → A 365-day calendar falls slightly short → Add 29 February in a year divisible by 4 → Skip century years not divisible by 400 → Keep the calendar close to the seasons

Also know

In the Gregorian calendar, 1900 was not a leap year, 2000 was, and 2100 will not be. Its average year is 365.2425 days. The question’s four-year answer is the general rule; the century exception matters whenever a particular year is given.