Pf(4) : controlled buffer and additional inputs

For the pf(4) question, are we allowed to use the controlled buffer gate?
Also are we allowed to create a bus of zeros as an input in addition to the existing given input?

You should not use the controlled buffer, as it gives a non-logical output if the control signal is “0”.
You’re thinking along the right lines. Think about how you can implement this functionality using basic logic gates.

You are not allowed to add inputs. If you need zeros, you can use a constant from the “Wiring” library.