2 is somewhat confusing to me too, because it doesn't appear that any of those species are insoluble in water. It is like a precipitation.. except there will be no precipitate (unless huge amounts are used). Nevertheless, it is not redox.
3 is a synthesis reaction but it is interesting. First, it is not a redox reaction. Not all synthesis/combination reactions are redox reactions. On a test I would probably choose none of the above, although I can make a case for it being acid/base neutralization (Lewis acid/base). I suppose I can stretch it out and call it precipitation, but there are no ions, so I would not choose that.
5 is correct.