Hi Boo, answers to 1.) Move/rename scoots the malware out if it's target location, thus making it harder for it to activate its self.
2. Delete, by its very name is self explanatory, it deletes whatever Avast! found.
3.) Repair fixes or attempts to fix the infected file by removing the malware from it.
4.) Scoots the whole file over into the virus chest, and corks it up so that it cannot be used.
As for the "Take no action" thing, I'm not sure but I think that is just telling Avast! to leave it alone, that you know it's there, and that you're happy with it. Will not be activated, I think means at this time.
♥ Wendy