Shamita's birthday was on 15 March 2020, which was a Sunday. If her husband's birthday was on 31 May 2020, on which day would it fall?

Days left in March: March has 31 days. From March 15th to the end of March is 31 - 15 = 16 days.

Days in April: April has 30 days.

Days in May until birthday: 31 days.

Total days: 16 + 30 + 31 = 77 days.

Divide by 7 (days in a week): 77 ÷ 7 = 11 with a remainder of 0.

Zero remainder means the same day: Since the remainder is 0, the day of the week for May 31st will be the same as March 15th, which was a Sunday.

Therefore, the answer is Sunday.

Thus, correct option is (b).