Find the odd number pair: 80 : 9, 64 : 8, 36 : 6, 7 : 49.

80 : 9 is the odd pair.

64 : 8, 36 : 6 and 7 : 49 pair a number with its square root or square (64 = 8^2, 36 = 6^2, 49 = 7^2).

In each pair one member is the square of the other.

9^2 = 81, not 80, so 80 : 9 does not fit.

So the pair without a square link stands apart.