Is 1900 A Leap Year? Why/Why Not?

No.

1700, 1800, 1900 are not a leap years, because the rule for our current calendar is that only century years that are a multiple of 400 are leap years.

The calendar tries to approximate the length of the "tropical year," which is a moving target. Because the earth is slowing in its orbit, the year is actually getting longer. The current calendar rules peg the year length at 365 days + 1/4years - 3/400years = 365.2425 days per year. Astronomers tell us the tropical year was about 365.24219 days in the year 2000.

It is convenient for some of us who write computer programs involving dates that 2000 was a leap year. That way, the program can work well through our lifetime and never have to know about the rule above.

Yes 1900 was a leap year, to figure out if a particular year was a leap year or not divide it by 4. If the date is evenly divisible by 4 then it is a leap year, if it isn't evenly divisible by 4 then it is not a leap year.