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A: which
This kind of grammar is called relative clause. The best option “which” helps to connect the clause "advice I follow" back to the idea of having a bath, indicating that the bath is the specific advice being followed.
1.which: used to refer to something. Ex. The church which has a red door is beautiful. Ex. I know the church which has a red door.
2.whose: used to refer to someone who owns something. Ex. The church whose door is red is beautiful. Ex. I know the church whose door is red.
3.It is not related clause.
4.It is not related clause. (Related clause is kind of similar to adjective, in your sentence, “taking a bath” is an advice being modified by the related clause “which advice I follow”. And here is a common use of what: I know what your name is. In my sentence, “what your name is” doesn’t modify “know”.)
We add comma before (and after) related clause, when the thing we refer to is specific. Ex. The bishop’s church, which has a red door, is beautiful. Ex. I know the bishop’s church, which has a red door.
thank you very much:）