An isosceles trapezium is a special case of a trapezium where both legs (non-parallel sides) have the same length. The base angles are also congruent.
The diagram contains a trapezium with both base angles (C and D) with the same tick mark, so they are congruent. Under this condition, the trapezium is isosceles and both legs have the same lengths, thus:
BD = AC = 31 cm
Answer: b. 31 cm
