Answer:
Hi there!, the best answer is B.) estábamos nadando
Step-by-step explanation:
The reason why this is the correct answer is bacause...
In the sentence, the girl was talking about the experience of Sandra and him/her friend (?) had when going to the river last week, until they saw the fog, which means there were two people, his/her friend, Sarah and him/her, which, the term, we or we were [insert verb here} in spanish is (we) "nosotros" or (we were) "nosotros estábamos [insert verb here].
Second thing was, they were swimming, or "nadando", in spanish, together.
So the best answer will be B.) "estábamos nadando"
I'm sorry if this explanation is to wordy, but it has the answer that you wanted with this detailed explanation
I hope this helps :)