As you suggest, it isn't possible to know the gender of the question without the pronoun and this is an error we deleted from the course quite a while ago but the system is taking its time to remove it.
There is an important meaning of a standalone 'oedd'. It is the affirmative (the 'yes') for the third person in the imperfect.
Oedd hi ar y bws ddoe? Oedd (Was she on the bus yesterday? Yes (She was))