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