The issue with this question falls into 2 main categories. The first is that its a rude question. The basic premise seems to be, "if you are an experienced ops person you have to know something about ps". Which may be true, but by asking it of an experienced operator you are hinting that you don't believe their resume. There is a much more straight forward way to see if they are lying on their resume, call their references and prior employers. For really good operations folks, the answer to this question might be "I haven't needed ps in so long I've forgotten how it works. I've automated away the process running abstraction and have a library of python/bash/salt/chef/esoteric top commands/etc that I find more useful than remembering ps flags". This question implies their experience in that regard is also suspect.

The other problem with this question is that, by its phrasing, it implies that there is a right answer. If I can name more ps flags than an other person, I'm a better candidate. Which when put that way, I think points out its faults. Maybe you don't intend for that to be true, but you'd have to fight your own cognitive biases pretty hard to not at least bias towards the person that can name 17 flags off the top of their head, or the one that teaches you a new flag combo you didn't know. Even though those things are likely not that good of predictors of a good candidate.

You've said what you really want to get to, is can they admit when they don't know something. If that is important to your hiring process (and I'd encourage you to validate that with data), ask them a question no one could know the answer to. Ask the same question to every candidate, and grade it purely pass fail. Did they say "I don't know" or not.