If you know for a fact that someone is not married, then the only word you would not use is Mrs.
If you don't know her marital status, then you can use Ms.
If you know for a fact that she is not married, then you can use Miss.
A common-law marriage (in the UK at least) does not generally lead to the use of Mrs, though some women might choose to change their surname to that of their husband, and use "Mrs" but it's personal choice.
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