In the following sentence why using
Was instead of
Wo is not possible?
Wo arbeitet Hans?
according to my book the only possible W-question is
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Arbeiten is (in standard German) an intransitive verb, i.e. it cannot have an accusative object; which is what you would be asking for with was. When asking for an occupation, the question is:
Als was arbeitet Hans? Hans arbeitet als Maurer.
With wo, you ask for the place. This can give you a hint about the occupation, but it is usually less specific, and it may also prompt a completely different answer:
Wo arbeitet Hans? Hans arbeitet auf dem Bau. Hans arbeitet in Frankfurt.
In colloquial language, Was arbeitet Hans? is sometimes heard, but it wouldn't be considered correct.
It is not true that wo is the only possible question word. You can of course ask:
Warum arbeitet Hans? Wie arbeitet Hans? Wann arbeitet Hans?