Answer:
McManus killed Keaton
Step-by-step explanation:
As we know that only one is saying the truth, let's analyze who might that be.
When we read the four statements, we notice that Fenster and McManus are saying exactly the opposite, so they can't be both lying. One of them is saying the truth, let's see who is.
If Fenster is saying the truth, Soze killed Keaton. But Soze said McManus didn't kill Keaton and he is lying (because just Fenster is saying the true), that means McManus killed Keaton. We have a contradiction here (we knew Soze killed Keaton), so Fenster can't be saying the true.
Therefore, McManus is saying the truth and the other three are lying. Let's see there's no contradiction here:
McManus said Soze didn't kill Keaton and that's true. Fenster is lying, so Soze didn't killed Keaton, that matches what McManus said.
Hockney said McManus was in a meeting when Keaton was killed, as that's a lie, McManus wasn't on a meeting so he could had killed Keaton. Finally, Soze is also lying so McManus did kill Keaton.
