Which of the following is not a characteristic of all parallelograms?
a. Opposite sides are equal.
b. Adjacent angles are congruent.
Answer: The characteristic which is not true for all parallelograms is adjacent angles are congruent.
Lets us understand the concept below.
In the figure, one can observe that adjacent angles of a parallelogram are acute and obtuse respectively.
∠A or ∠BAD and ∠C or ∠BCD are obtuse, ∠B or ∠ABC, and ∠D or ∠ADC are acute.