There are 365 days in an ordinary year and 366 days in a leap-year.

365 divided by 7 (a week period) yields a remainder of 1, thus the same date after an ordinary year would fall on the next day of a week;

366 divided by 7 (a week period) yields a remainder of 2, thus the same date after a leap-year would fall two days after in a week;

Thus 3 years, 1988, 1989 and 1990, will accumulate 2+1+1=4 days, plus 2 days of difference between 19th and 21st, which gives total of 6 days of difference (6 days shift). Thus, 21st of September 1990 was Saturday+6 days=Friday.

Answer: E.
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19th September
1987: Saturday
1988: Monday (due to leap year)
1989: Tuesday
1990: Wednesday

Hence 21st September = Friday. Ans. E