Correct Answer - Option 1 : Monday
To find: 4th day before the 21st day of the month = 21 - 4 = 17th day
CONCEPT: Days of the week repeat after every 7 days i.e. 3rd day, 10th(3 + 7) day and 17th(3 + 14) day would be the same.
So, the 17th day would be the same as the 3rd day and therefore, it would be Monday.
Hence, the 17th day or 4th day before the 21st day of the month is Monday.