Which is an open-ended question that could be used to start a discussion?

Why do you think the bus should run more often?

How many people take the bus?

Do you take the bus?

What time does the last by leave?

Why do you think the bus should run more often?

This is the only question that gets you really thinking. All of the other ones can simply be replied with a short, simple answer. A discussion can't really be created with the other questions. This one is a question that could have many answers and be detailed. It could make the discussion go on longer, while the other ones would end quite quickly.