If it was Monday on January 15th 1989, what day it would had been on January 20, 1990 ?

The correct answer is (b).

The day of the week repeats itself every 7 days. So, if it was Monday on January 15th 1989, it would be Monday again on January 20th 1990.

Option (a) is incorrect because Sunday is the day before Monday.

Option (c) is incorrect because Tuesday is the day after Monday.

Option (d) is incorrect because Wednesday is the day after Tuesday.