Thank you for your question and for citing your source.
As you have already surmised, the word "yesterday" rules out the present-tense forms used in (a) and (b).
I agree with you that (c) is the best answer. I will also say that (d) is grammatically correct, technically, but awkward. We wouldn't normally express this in the passive progressive. In addition, the fact that the speaker expresses an opinion about the beauty of the boats indicates that he or she personally witnessed the event, whereas "were being seen" implies that other people, not the speaker, were doing the seeing.
1: A dark-haired woman in a green dress could be seen leaving the bank just before the robbery. She was very pretty.
2: A dark-haired woman in a green dress was seen leaving the bank just before the robbery. She was very pretty.
3: A dark-haired woman in a green dress was being seen leaving the bank just before the robbery. She was very pretty.
If I were an actual eyewitness to the event, I would only say (1). I could see her. I could also use the first sentence if I were not an actual witness, but not the second. I would have to reword it along the lines of
1a: She has been described as being very pretty.
Example (2) could be reworded in the active voice as
2a: Some people saw a dark-haired woman in a green dress leaving the bank just before the robbery.
But again, I'm not a witness. I can't say whether she was pretty or not. I can only echo the descriptions of actual witnesses, as I did in (1a).
Do I really have to talk about (3)?