Correct Answer: Sunday

Solution : Given:
13th of December 1969 is Saturday.

Let's calculate the total number of days from the 13th of December 1969 to the 9th of July 1978 –

Remaining days of 1969 = 18
Number of days from 1 January 1970 to 31 December 1977 = (8 × 365) + 2 = 2922 (2 leap years)
Number of days from 1 January 1978 to 9 July 1978 = 31 + 28 + 31 + 30 + 31 + 30 + 9 = 190
So, the total number of days from 13 December 1969 to 9 July 1978 = 18 + 2922 + 190 = 3130
Now, on dividing 3130 by 7, we get 447 as the quotient and 1 as the remainder.

Therefore, add 1 to Saturday, Saturday + 1 = Sunday

So, the 9th of July 1978 will be Sunday. Hence, the third option is correct.