i believed i explained this to you before. remember, we look for symmetry about vertical and horizontal lines first, then any other lines that we can fin symmetry about.

any vertical line that cuts the region into equal halves is x bar

any horizontal line that cuts the region into halves is y bar

for a circle centered at the origin, the axis do this. so we have y = x = 0 giving the desired lines

EDIT: oh sorry, we're not dealing with a full circle, but the essence of my explanation is still true, so it's not specifically x = y = 0 here, what would be the point of the question then?

the line x = y cuts the quarter-circle into two symmetric halves, that's why they got that answer