"The reactions from the public are good."
Translation:Les réactions du public sont bonnes.
When public is a noun, it is invariable: le public.
Le public aime le nouveau musée. → The public likes the new museum.
Public can also be an adjective and as such it agrees with the noun. The feminine form, publique, is always an adjective.
Où est le jardin public ? → Where is the public garden?
Ses enfants vont à l'école publique. → His children go to public school.