In a), either form is OK in either sentence, Emanuelli.
It was nice meeting/to meet you.
It was nice talking/to talk to you.
They are minor variations in a common structure. The infinitive may tend to suggest the complete act, while the -ing form may tend to emphasize the experience of performing the act.
Many verbs take either form (and many others do not!), but only in a few does it make a significant difference to the meaning (for instance, 'remember doing/to do').
"What you need to do is take the opportunity." -- the subsequent 'to's in a parallel structure (S-C) or series may be elided if the meaning is clear: 'what you need to do is wash the dishes, clean your room, and pick up your clothes'.
"All she does is watch TV" -- this one stumps me, because I would have thought (until I tried it in my mind's ear) it should be 'all she does is watches'. There is no call for an infinitive here, yet I cannot account for the seemingly more natural unmarked verb form. If I write 'what she does is watches TV all day and never cleans the house', the parallel structure falls into place, but in your sentence--?
Maybe Casi willl help us out.