You're correct, you simply reverse/switch the two words and add the negative "n't". He is happy >>> Isn't he? They are disappointed >>> Aren't they? We are satisfied >>> Aren't we?

Yes, the rule for question tags in standard English is simple. Use a negative question tag after a positive statement, and a positive question tag after a negative statement. In informal usage, there can be some regional variations. For example, Canadians are fond of using "Eh?" as a question tag after both positive and negative statements.

You’re right, A is the usual form, turning the statement ‘he can play soccer’ in to a question using the negative of the verb : ‘can’t he ?’. However there is also the possibility of using the version in C in some situations. This may apply when there is less certainty about the answer - it is more open to both positive ‘yes, he can play soccer’ and negative ‘no, he can’t play soccer’. The more common form in A assumes that he can play soccer, so the answer will usually be ‘yes, he can play soccer’, although there may be a ‘concession’ added : ‘but he can’t play this week because he’s injured’.