Question

which of the following is a product of an acid -base reaction ?
a.base
b.acid
c.salt
d.fire
?

Answer 1

salt

Answer 2

It may help to remember that an acid-base reaction is called a neutralization reaction, because it neutralizes -- destroys -- the reactive nature of both the acid and the base. That immediately rules out a and b. d is somewhat silly, because fire is a phenomenon, not a substance. That leaves c right away. But also you can realize that salt, e.g. table salt, is a pretty harmless, unreactive substance. So that's a plausible outcome for a neutralization reaction that is going to destroy the reactive nature of each of its reactants.

Note that this is assuming, which is reasonable for your question, that you have stoichiometric amounts of acid and base -- they use each other exactly up. If they do not, it is entirely possible for the result to be an acid or base. For example, a little bit of NaOH (a strong base) added to acetic acid (an acid) will produce acetate anion (a base). But I doubt your interlocutor is looking for this level of subtlety.

Answer 3

salt and heat so next to c.salt d.fire could also be accepted

Answer 4

if the teacher is in a good mood.