The answer is compound (b).
A mixture is two substances mixed physically, but not chemically, so it cannot be A, because two atoms combining is automatically a chemical change. An emulsion is a dispersion of water droplets, so it has absolutely nothing to do with atoms combining. An isotope is an atom of an element with a different number of neutrons, which does not deal with two atoms being bound together.
A compound, however, matches the definition perfectly!
Hope this helps!
