Final answer:
The most appropriate way to say 'hi' to Sylvie is 'Salut' (Hi) in French.
Step-by-step explanation:
The most appropriate answer to say 'hi' to Sylvie would be 'Salut' which means 'Hi' or 'Hello' in French. This is a common greeting used among friends and acquaintances. Other options like 'Bonne nuit' means 'Good night' and is used when saying goodbye in the evening, 'ça va?' means 'How are you?' and is a question used to inquire about someone's well-being, and 'toute à l'heure' means 'See you later' and is used as a farewell.
Learn more about Greeting in French