what does 'bladder' even mean here?

Yes, this is talking about the bladder that holds your urine. Some people experience difficulty urinating around other people - particularly men in public restrooms where urinals are commonplace. This is colloquially referred to as being 'pee shy'. This poll is making a joke about being pee shy.

Yes. This refers to a condition known as "shy bladder", when a person is unable to pee when others are nearby (which can be a problem when using public restrooms). "Confident bladder" is not a common expression that most people would know, but some people, including this redditor, have used it to refer to not having any problems peeing while others are around (since "shy" and "confident" are opposites).

Without any context, it's hard to say for sure. But I think ... yes, the actual bladder that holds your urine. There's a term "shy bladder" that typically refers to when a man can't pee if other people are (potentially) looking at him. So, I think the poll is mostly asking how easy it is for you to pee at urinals in public restrooms, though there are likely other situations this could come into play. I could be wrong, though. There's not much information to go on here.

Yup. Thats hilarious. Some people have reason to doubt their bladders. Admittedly a question that would make more sense would be, “How confident are you in your bladder?” But how confident is your bladder is another - somewhat amusing - way to pose a similar question.

No idea. What is the context?