Final answer:
In a trapezoid with congruent diagonals, consecutive angles along the same diagonal are supplementary. Since angle D measures 71°, angle B, being the consecutive angle to angle D, measures 109°.
Step-by-step explanation:
The question involves determining the measure of angle B in a trapezoid with congruent diagonals. Since one of the properties of a trapezoid with congruent diagonals is that the consecutive angles between a diagonal are supplementary (meaning they add up to 180 degrees), we can find the measure of angle B. Given that angle D is 71°, angle B must be supplementary to angle D if they are consecutive angles along the same diagonal. Hence, the measure of angle B is 180° - 71° = 109°.