That means it's offered for sale at a very very very very good price.
The price may be so good because the owner doesn't know what it's actually worth,
or because they're having a super-duper temporary sale, or because it's something
you personally need very badly so it's worth a lot more to you personally.
In any case, it's called a 'steal', because when something is stolen, the stealer
gets what he wants for a very very very low price, and this is just like that.
The answer is 3 because it starts with “on” which is a preposition
