Where should citations be placed relative to punctuation (e.g., full stops and commas)?
As others have mentioned, all sentences should be grammatically correct. The citation marker of the form [X] may play the role of a noun, or of silent word.

1, 4 and 11 are never correct - they put the citation into the next sentence, where it doesn't belong. Similarly, 8 and 9 put it into the wrong phrase of the sentence. 

3, 6 and 9 combine words incorrectly. Spaces can't just be ignored.

That leaves 2, 5, 7 and 10. These are all used, except that the exact form of 7 will depend on the referencing style in use. You could have 'According to Smith [3]', but not 'According to Smith [Smith, 2017]', The latter would become 'According to Smith [2017]'. Also, for 10 you need to think about how long the second phrase is.