Ooh, this is rather a contortion, isn't it?
First let's be realistic. The given answer is absolutely the way the question would be phrased in real life. It is both clear and concise. We must remember that the main purpose of the spoken language is to make ourselves clear, and conciseness is the best means to that end.
Now look at the event. It was a robbery. That means, almost inevitably, that it was a pretty quick happening, unlike a burglary. Therefore to use the continuous when describing it (no.1) would be inappropriate.
If the police wanted further detail after the initial response of 'I was waiting at the bus stop', they could then elaborate by using further questions, none of which would be your alternatives because there would be too much repetition.
So the given answer is the only one. Now let's suppose it wasn't a robbery, but a burglary or streetfight. In other words, something that lasted a sufficient time for you to do something whilst it was happening. Your alternative answers would now be more relevant, but only 2 b) is correct as it stands. In 1) and 2 a) you'd need to replace the word 'when' with 'while' because 'when' is used for a short event (eg: robbery).
'What were you doing while (or as) the burglary was taking place?'
'I was watching TV the whole time.'
Note: 'was taking place' can be replaced with 'took place' as it's all over now.
But no.1 still sounds weird: 'What did you do when you saw the streetfight taking place?' would be more natural and gain you the information you're after.
Now my head's spinning. Perhaps I should just have answered your question with 'No'.
- For Teachers