Implementing a boolean function using a decoder
The reason you are getting incorrect expression is because your K-map is incorrect. Look carefully, 0110 (6) and 0111(7) should be zero, not one.

If you construct the K-map properly, you will arrive at the same expression ie

Q = W + Y'


which using Demorgans rule becomes

Q = (W'.Y)'


This corresponds to (01)' . But since your outputs are active low, this is no problem.