The correct answer is (c) perpendicular
Easiest explanation: θ = 90 degrees ⇒ cos θ
a1a2 + b1b2 – c1c2 = 0
Relation between the planes a1x + b1y + c1z + d1 = 0 and a2x + b2y + c21z + d2 = 0, if their normal are perpendicular to each other is a1a2 + b1b2 + c1c2 = 0.