So there is an unused input for the bottom NAND gate (even though it is not shown in the picture) and the question wants us to connect it while preserving the logic expression. I know when we connect it to the ground, the lower NAND gate never outputs 0. So I assume we are not going to connect it to the ground. But when we connect it to the +5V supply or E, we can get 0 or 1 as an output. Connecting it to E makes it work like it is a two-input NAND gate. I know I have to choose one of them but which one. I would appreciate it if I can get an explanation of what I should do when I come across these kinds of problems.
By the way I assumed we needed to connect it to E and made these answers (I would appreciate feedback to these also):
For the truth table I used this logic expression: