Formal:
* Buenos días, señora: This is the most formal and respectful way to greet a woman. It literally translates to "Good day, madam."
* Buenos días, señorita: This is also formal but slightly less formal than "señora," and would be used for younger women or women you don't know well. It translates to "Good day, miss."
Informal:
* Buenos días: This is a general greeting for "Good morning" and can be used for both men and women, but it's less formal than the options above.
Using the person's name:
* Buenos días, [name]: This is a more personal greeting and would be appropriate if you know the woman's name.
The best choice depends on the context and your relationship with the woman. If you are unsure, it's always best to err on the side of formality and use "Buenos días, señora."