If you ask me:
1a is wrong
1b is fine
1c is fine too, only higher in register and consequently less common in colloquial speech.
2a is fine
2b is used and will work, especially in conversations, where a context is available. The same holds true for sentences like "Sono uno e ottantasette": – "Quello al centro è un gigante!" – "Eh sì, sarà almeno uno e novanta".