With regard to grammar, both are technically 'correct'. There are no grammatical mistakes in either sentence. With regard to 'meaning', however, and the context in which you would use such sentences, the second one is extremely unlikely. The first sentence is fine. We use the 'going to' form to indicate a plan or arrangement, so it would be normal to say 'I'm so excited' about something that are expecting to do. 'Will' isn't normally used in this situation, so although 'I will go sailing next Sunday' is not a grammatically incorrect sentence in itself, it doesn't go with the idea of looking forward to something. Does that make sense for you, Lela?

a. is correct.

A is correct. To fix B, you can say.. "I will be going sailing ..." In any case, A sounds more natural to me.

both correct i think..