Implementing Logic Using 2x4 Decoder (Confusions with Non/Negated Inputs/Outputs)
I may be wrong, but it looks like the A and B of the decoder truth table corresponds with B and C in the top table (A being wired to the enable)
In other words, the top tables A, B and C should be seen as signal names, rather than the names of pins on the decoder.
So signal B goes to pin A, signal C goes to pin B, and signal A goes to enable pin.
Maybe the original question was badly set out.