If it was Sunday on 1st September, 2013, then what was the day of the week on 1st September, 2021? 

We know that: The number of odd days in a non-leap year is '1'. The number of odd days in a leap year is '2'. 2016 and 2020 are two leap years between 2013 and 2021. So, the number of odd days from 1st September, 2013 to 1st September, 2021 = (1 + 1 + 2 + 1 + 1 + 1 + 2 + 1) days = 10 days The number of days from 1st September, 2013 to 1st September, 2021 = 10 days The days of the week repeat itself after 7 days, so, (10÷7) , we get 3 as remainder. So, the number of odd days = 3 As, it was Sunday on 1st September, 2013, so, it was Wednesday on 1st September, 2021. Hence, option d.