In casual conversation, both hold on and hold up can be used to mean “wait.” In your sentence, if only one answer is right, I’d assume it would be “hold on.” However, in America, you could say ‘hold on a second’ or ‘hold up a second’ and no one would really question it.

We use both 'hold on' & 'hold up' to mean wait. I don't know anyone who says 'hold out' to mean anything other than eg hold out that item to look at etc I'm Australia and to me 'hold up' sounds very American. I don't know, that's just my experience