I think both 'a' & 'b' are OK. Am I right?


Yes, you're right, but I mostly agree with the past continuous

1. Once you use the past simple :

The sentence means: When Nader was in Sharm El-Sheik, he used to sunbath a lot. = past activity no longer he did

2. https://www.grammaring.com/past-continuous-for-temporary-habitual-actions-in-the-past

Past continuous for temporary habitual actions in the past

The past continuous is used to express repeated or habitual actions in the past that were temporary:
At that time, she was meeting him twice a week.
When I lived in London, I was getting up at 5 am to be able to get to work by 6.