The problem is that the AG doesn't say what those "most common" abilities are--it kind of assumes that you already know from playing D&D. I don't think it's a huge problem, but as you can see, it's one that can lead to confusion.
Yes, this is my point. Technically it does list, under each skill's description, the "most common" one, but nowhere is it indicated that this is part of the calculation. In fact, technically it doesn't even say abilities should be used at all when calculating passive scores, it simply implies it by listing it in the ability checks section.
The character sheet has three baked-in passive skills on it but no guidance as to how to calculate them. If it's meant to be Passive Stealth (DEX), Passive Insight (WIS), and Passive Perception (WIS), then it should say so. And if they aren't meant to have an ability associated with them, what's even the point of having them on the sheet? It is no more useful than just looking at my skills. Doesn't help me or the Narrator.
Honestly, there are a number of things on the sheet that aren't very clear to me (this isn't just an A5e problem, though). I'd love to have someone write a "how to" article of sorts that demonstrates what is meant to go where and why.
I’m really sorry, I’m not trying to be obtuse, I promise, but I honestly don’t understand the question.
I mean no offense, but it really just feels like you are intentionally unhelpful sometimes, notably every time I (or others) have asked for rules clarifications. I know you say that you are not, and I want to believe you, but it is very frustrating. You asked for options, so I provided two and you didn't choose either of them, nor did you address my additional thoughts. I really feel like I provided a wealth of details and you just... didn't read them and now I am just repeating myself over and over. Faolyn has provided a simple answer, which is great, but it doesn't address whether or not that was the intent of the designers, which is really what I am after. Community speculation is my second least favorite thing (Jeremy Crawford's vague and unhelpful responses are my least favorite) about O5e and I have really been hoping for some real clarity with A5e that I have, so far, not gotten.
It’s what you personally use most commonly. The AG doesn’t know you or your character. It’s just a convenience thing.
But if I've never played before, I also don't know my character or what is most common. So it's not very convenient to have no guidance. Additionally, it's up to the Narrator to decide what skill to use, yes? So I, as a player, would have even less idea what might be "most common" to the particular game I'm in until I'm well into it.
At the end of the day, whereas in O5e passive skills were a nice shortcut that could just go on the DM screen for reference, they seem like a lot more hassle and a lot less useful in A5e. There are, essentially, 120 passive skills in A5e. Since any ability can be used for any check, that's 6 per skill and there are 20 skills provided on the sheet. Hardly seems like it's saving any time for a Narrator...