Concept: The question tests on Average

Solution:
Let the marks be 5x, 6x, 7x and 8x

Let 5x = 80, then x = 16. We see that 7x = 102, which crosses the max marks of 100

let 6x = 80, then x = 40/3. 7x then becomes a fraction, and we are told that all the marks are integers.

Similarly, if 7x = 80, we get marks as fractions.

If 8x = 80, then x = 10 and the marks are 50, 60, 70 and 80.

The average is therefore (50 + 60 + 70 + 80)/4 = 65

Option C