As the conjugation is a subjunctive, one can eliminate :
- Il paraît
- Il est certain
- Il croit
(and by the way, 'Il croit' means 'he believes' : it's not an impersonal form like the others).
Note that all those 3 forms would be acceptable with an indicative (future because of the 'bientôt'), for example :
Il paraît que les trottinettes seront bientôt interdites sur nos trottoirs
So the only valid one is :
Il est possible que les trottinettes soient bientôt interdites sur nos trottoirs.
Here are some other elements that would fit in here :
- Il semble
- Il est à craindre
- Il est souhaitable
As for the why, it's dictated by usage, and there a lot of exceptions but :
- For a possibility or a wish, it's mostly subjunctive mood.
- For something certain or obvious, it's mostly indicative mood.