Which of the following quadrilaterals is not always a cyclic quadrilateral?

1. square

2. rhombus

3. rectangle

4. an isosceles trapezium

Given,

ABCD is a rhombus.

In a rhombus,

All sides are equal, but angles are not necessarily equal to 90°.

Opposite angles of a rhombus are equal.

So,

∠A = ∠C and ∠B = ∠D

But,

∠A + ∠C ≠ 180° (always)

Hence, opposite angles of a rhombus are not always supplementary.

Therefore, a rhombus is not always a cyclic quadrilateral.

Hence, option 2 is the correct option.