The correct answer is who works at that desk.
There are two types of clauses: subordinate and independent. Independent clauses, as the name itself suggests, are complete and can exist on their own. In the example above, the independent clause would be <em>The woman is absent today.</em>
On the other hand, subordinate clauses are incomplete thoughts - they need to be supplemented by other words in order to be grammatically correct. In the example above, if you just write the clause <em>who works at that desk </em>and leave it at that, it is definitely grammatically incorrect. But, if you write that entire sentence, then it will make sense.
