I would say people would use the first one for both writing and speech. In modern English, writers do not use formal writing for fictional books. Also, I would probably keep the tenses the same, "The woman I talked to was very beautiful". Although she is still beautiful now, we would say she was beautiful because we are talking in the past tense, it sounds weird to have the past for to talk and present for to be.

Hi Victor, d) is grammatically correct. We use "whom" to emphasise on the person who received an action. In this case, the woman received the talking action from you. If you would like to use "who", then you can write : I spoke to the woman who is very beautiful. Hope this helps. Cheers, Lance

Hi Vítor! In casual, informal spoken English, all four of your examples are fine. When you're talking to friends, they will understand exactly what you mean if you use any of them. (By the way, your A and B are exactly the same.) In formal written English, D would be correct. This should be "whom" because the woman you're referring to is the object of your sentence and not the subject (your subject is "I"). There would also need to be another change: "is" should be "was" because, in this example, your verbs need to match their tenses. Since you "talked" to her, she "was" very beautiful (obviously, she probably still "is" beautiful right now, but you're not currently looking at her to be able to confirm that, so it's not present tense). So your final sentence should read: "The woman whom I talked to was very beautiful." I hope I've helped. Please let me know if you still have questions.

'Whom' is not informal in any way. It would be exceedingly rare and odd to hear your d in an informal context.