Translation:Žofie thinks that spiders are beautiful.
If you are asking about simple present vs. present continuous, I would offer that using "is thinking" suggests to me that, at this moment, Kateřina is having that thought about spiders. When the simple present is used, it suggests that this is a more "permanent" thought that she has about spiders which persists over time.
That is an interesting point, but in English in my opinion there is not much difference in this context between meaning of the simple and continuous presents and they are pretty much interchangeable. If the Czech would make a significant differentiation, could you say how "Žofie is thinking..." would be expressed, as it is not currently accepted as an answer in this exercise? Thanks.
Thanks for your reply, but in that case how would a Czechs person express "Žofie is thinking that spiders are beautiful"? Or would a Czech never intend to express that? I realise that we English speakers have developed three present tenses ("I think", "I am thinking", "I do think", with very subtle nuances of differentiation) which can make translations difficult.