To solve this question, the user needs to know the number of days in each month and the concept of leap years.
Option A is incorrect because June only has 30 days, and there is no 31st June.
Option B is correct. Although February usually has 28 days, in a leap year, it has 29 days. February 29th, 2000 was a valid date.
Option C is incorrect because April only has 30 days, and there is no 29th April.
Option D is incorrect because August only has 31 days, and there is no 31st August.
Therefore, the answer is: A. 31st June 1999.