Where do you see yourself in X years?
I really enjoy aspects X, Y, and Z of my job, and I'm hoping that in the future I can start doing more of A and B, and perhaps even grow into more of a supervisor (or some relevant next step).
That's is true statement. It doesn't address their specific timeline, but it's honest. Likewise things like afirming that you work hard and you believe you give your full effort for every project you are assigned. If they ask something specific about whether you are thinking of leaving, in those shoes I might say something like, "I'm very happy here at Acme Widget and when I think of my future, I mostly imagine myself working here until I retire, whenever that may be, assuming the compensation and opportunities here remain strong." Again, true (from the sounds of it). If you are considering leaving (not for FIRE, but for another job), there is some risk involved, but it might be a great time to say, "I really love it here and would love to stay here long term, but I know there are other jobs out there that pay more. I don't want to leave, but is there any chance of renegotiating salary or benefits (or ask for something specific like working from home for one day, a 4/10, or whatever)? Something like that would definitely make it easier to ignore those outside opportunities that I keep hearing about." Again, there is risk in that, so you have to decide if the gamble is worth it. If not then just ignoring the question part and telling them that you are happy they are pleased with you, and telling them you are pleased with your current situation will probably be sufficient.