The oxidation number of an element in a compound is the charge it would have if all the electrons in the bonds belonged to the more electronegative atom. In this case, we can start by finding the oxidation number of Cl since its value is more common and well-known. The compound is K3ZCl6, which means that the total charge of the compound must be zero since it is not an ion. We know that the potassium ion (K+) has a charge of +1, and there are three of them, so the total charge from potassium is +3. The chloride ion (Cl-) has a charge of -1, and there are six of them, so the total charge from chloride is -6. Therefore, the oxidation number of Z can be found by solving the equation:
(+3) + (oxidation number of Z) + (-6) = 0
Solving this equation gives us the oxidation number of Z, which is +3. Therefore, the correct answer is option (B) +3.