(a)
The coordinate of the point of intersection of line $y = x$ and circle $x^{2} + y^{2} - 2x = 0$ is $A(0,\ 0)$ and $B(1,\ 1)$
$\therefore$ Equation of circle with $AB$ as its diameter is
$x(x - 1) + y(y - 1) = 0$
$\Rightarrow \ \ \ x^{2} + y^{2} - x - y = 0$