Answer:
today
Step-by-step explanation:
Today is the only one that isn't a signal word for dependent clauses. It can be placed before or after independent clauses. For example:
Today, I went to the store.
As you can see the word today does not mean that the sentence can't be on its own. It makes sense either way. If you wanted, you could just say:
I went to the store.
And it would have almost exactly the same meaning depending on context.