(d) intersecting at $(a, b)$
The line $x = a$ is vertical and $y = b$ is horizontal; they intersect at the point $(a, b)$.
$x = a$ is a vertical line parallel to the y-axis, and $y = b$ is a horizontal line parallel to the x-axis. Every vertical and horizontal line pair intersects at exactly one point. At that point, the x-coordinate is $a$ and the y-coordinate is $b$, giving $(a, b)$ — not $(b, a)$. Option (d) is correct.