Given equation: $xy + yz = 0 \Rightarrow y(x + z) = 0$
This represents two planes:
1️⃣ $y = 0$
2️⃣ $x + z = 0$
Normal to first plane = $(0,1,0)$
Normal to second plane = $(1,0,1)$
Their dot product = $0(1) + 1(0) + 0(1) = 0$,
so the planes are perpendicular.
$\boxed{\text{Answer: (D) A pair of perpendicular planes}}$