Answer:
Hola, buenas.
Explanation:
A great way to greet a friend in a non formal way would be Hola, buenas, the other options are like see you later, see you soon, and Good night, so the most casual would be Hola buenas, which roughly translates to Howdy, in this case that would be the correct answer because the other aren´t greetings and buenas noches is too formal.
