The sentence with answer A is perfectly correct. Is it the last comma which bothers you? Actually, it does need to be there.

Adding the parentheses in the sentence really makes it clear!! Thanks peter :D "As soon as a base reacts with an acid, a salt (and sometimes water) is formed."

First of all, I don't think the sentence is true. An acid has an excess of [H+] and a base has an excess of [OH-]. When the two react you get a salt (not necessarily NaCl) and water (H20). I can't think of a way you can have that reaction without creating water. The sentence is strange. To be true it should read "As soon as a base reacts with an acid, a salt and water are formed." If you wanted to keep the "and sometimes water," I would write it: "As soon as a base reacts with an acid, a salt (and sometimes water) is formed." If you used commas around "and sometimes water," a person would have to diagram the sentence to figure out what means.