If the time gap between the original sound and reflected sound received by the listener is around 0.1 s, only then the echo can be heard.
The minimum distance travelled by the reflected sound wave for the distinctly listening the echo
= Velocity of sound ร Time interval
= 344 x 0.1 = 34.5 m
But in this case the distance travelled by the sound reflected from the building and then reaching to the girl will be (6 + 6) = 12 m, which is much smaller than the required distance. Therefore, no echo can be heard.