What should an 18-year-old girl do who lives in a foreign country and wishes to wear the hijab, but her parents have previously refused this, and she fears their refusal again if she tries to approach them about it once more?

An adult woman is obligated to cover her entire body from non-mahram (definition) men. This is by consensus among scholars, except for the face and hands. As for the face and hands, the more preponderant of the two scholarly opinions is that they must also be covered. It is not permissible for her family to prevent a girl from wearing the hijab (definition); otherwise, they would be sinful. Nor is it permissible for the girl to obey them in this, for the Prophet, peace and blessings be upon him, said: "There is no obedience in sin; obedience is only in what is right." And they must be keen on her covering and chastity, and saving her from the Fire, for Allah, the Almighty, says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones." Therefore, it is obligatory to wear the hijab, adhere to it, and not obey anyone in abandoning it.