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 this with the explanation written 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.