Which of the following statements is not true ?

The correct answer is: A kite becomes a rectangle if its opposite angles are equal.

Let us evaluate each given statement step-by-step using geometric properties to identify which statement is not true.

1. Analyzing the first statement: "A kite becomes a rectangle if its opposite angles are equal."
A kite is a quadrilateral with two pairs of equal-length adjacent sides. By definition, a kite has only one pair of opposite angles that are equal (the angles between the non-equal sides).
If both pairs of opposite angles of a kite are made equal, the kite becomes a rhombus (since all four sides are equal in a kite where both pairs of opposite angles match), not necessarily a rectangle. A rectangle requires all four interior angles to be right angles (${90}^{°}$), which is not guaranteed simply by having opposite angles equal. Therefore, this statement is false (not true).

2. Analyzing the second statement: "A rectangle becomes a square if all its sides are equal."
A rectangle is a parallelogram with four right angles (${90}^{°}$). A square is a special quadrilateral that has four equal sides and four right angles. If a rectangle has all its four sides equal in length, it satisfies all properties of a square. Thus, this statement is true.

3. Analyzing the third statement: "A rhombus becomes a square if all its angles are equal."
A rhombus is a quadrilateral with four equal sides. If all interior angles of a rhombus are made equal, each angle must equal $\frac{360°}{4}={90}^{°}$. A quadrilateral with four equal sides and four right angles is a square. Thus, this statement is true.

4. Analyzing the fourth statement: "A parallelogram becomes a rectangle if all its angles are equal."
A parallelogram has opposite sides parallel and equal. If all four angles are equal, each interior angle must be ${90}^{°}$. A parallelogram with four right angles is by definition a rectangle. Thus, this statement is true.

Conclusion:
The statement that is not true is: "A kite becomes a rectangle if its opposite angles are equal."