"but which" vs "therefore they”

Firstly the meaning is different. "Therefore" is used to introduce a conclusion from an argument. "But" is used to coordinate a contrasting idea.

Grammatically the two possibilities are different. The pronoun "which" would seem to refer to "structures". The pronoun "they" would most likely refer to "architects". Moreover using "Therefore they" would create a new sentence. Using "but which" would not.

I think the test just wants to you choose ", but which" (with a comma an lowercase b) to avoid a comma splice. It would be possible to have ". Therefore they" (with a full stop and uppercase T).

The meaning of "Therefore they" is also less likely, as the first sentence doesn't present an argument or reasoning for why the architects have intrigued the public.

The choice is logical. Does B follow from A, or does it go against A?

If it follows, it's a therefore.

If it goes against, it's a but.

The question pretty much tests if you understand such a complex sentence.

Read the sentence, focus on what it's saying, and if you get the meaning, then the answer should be clear.

Don't cheat! You wouldn't learn.

(sentence structure hint 1)

 Who or what has intrigued and excited the public?

(sentence structure hint 2 - answer to hint 1)

 It's the "various spaceship-like structures" that have intrigued and excited the public.

(sentence structure hint 3 - what we know)

 The structures are spaceship-like, they seem to defy the laws of engineering, and they have intrigued and excited the public.

(logical relationship hint 1 - compensation)

 Is that an advantage and disadvantage that balance each other out?

(logical relationship hint 2 - expectation)

 Would you expect the public to be intrigued and excited by those structures, or is it surprising that the public is intrigued and excited even though the structures look like that?

(final answer)

Well of course the public is excited by such cool structures, who wouldn't be? And there's no disadvantage.

It's a therefore.