'which' refers to the whole sentence (first part), while 'what' or 'that' normally only to the previous word.

Hey Tomas, The first one is correct. ............, which............. When you use "which" like this with a comma, it is almost like an afterthought. It gives extra information. Normally, relative pronouns modify nouns, however; you modify the whole sentence here. It's a non-defining relative clause. I didn't understand the sentence which she wrote. (We modify the sentence here) He passed all his exams, which we didn't expect at all. ( Here, the whole sentence is modified. ) Hope this helps! Feel free to check my profile :)

Hi Tomas! The first sentence would be correct: She came without her husband, which was quite a surprise.

Use should "which." "What" doesn't really work in this sentence.

1）is right