Comma

Not a.

B can be correct if you want to add additional information about her son's name.

Mrs. Tan and her son John are at the party (without any commas) is also correct to say which one of her sons was there.

Thanks. For me, (b), I got a question to ask.

(b) Mrs Tan and her son, John, are at the party.

If we put two comma as like above sentence, does it mean:

(i) Who is refer to "John" ? Mrs Tan and her son ? or her son?
(because we put two comma here, isn't make us confused?)

The two commas show that the information inside those commas is extra information. It is about whatever comes immediately before it.

I took my dog, Maggie, for a walk. This tells that "Maggie" is the name of my dog.

I, Grammar Geek, took my dog for a walk. This tells you that "Grammar Geek" is my name.