Hi Soufiane,
I am not sure I understand the question correctly, as I don't know that the full scenario is.
Anyway, please keep in mind that — when answering a question in the PMP exam — you need to pick the best answer among the options provided. Sometimes, more than one option may be reasonable, but one of them would be better than the other(s). Based on your description, the update to scope baseline was not an option, so you can take that for granted, and concentrate on the options provided. Try to eliminate those that you are sure will not need any updates in the specific situation you are in, and then I am confident that the update to "requirements documentation" will come up to you as the best option.
I would also recommend to take a look at the reasons why the other options are wrong: that helped me a lot in gaining a deeper understanding of the question and how to better approach it.
Gabriella