Right answer is (a) perpendicular
For explanation I would say: Relation between the planes a1x + b1y + c1z + d1 = 0 and a2x + b2y + c21z + d2 = 0, if their normal are parallel to each other is a1 : b1 : c1 = a2 : b2 : c2 ⇒ \(\frac {a1}{a2} = \frac{b1}{b2} = \frac{c1}{c2}\).