The point(a, a) has both its x and y coordinates = a.

Therefore we get, x = y = a

A. Point on x-axis is of the form (x, 0)

B. Point on y-axis is of the form (0, y)

C. Point on line x = y is of the form (a, a)

D. Point on line x + y = 0 is of the form (a, – a)

Hence C is the correct option.