If want to give explination of why i went to bed
so use because.
I went to bed because I was tired
that's right or not?
The blank is provided after 'I was tired'. what is expected is the result of this. therefore the answer must be 'so', which denotes the result. if the blank had been given like this:
I went to bed ---------- I was tired
then, the answer would have been 'because', which denotes the reason.
When answering questions, it is better not to change the structure or syntax of sentences. try to find answers that suit the given structure. that is the expected method. isn't it so, tdol?