dinner = 저녁/ 저녁 밥/ 저녁 식사 (1) 저녁 뭐에요? (2) 저녁 밥 뭐에요? (3) 저녁 식사 뭐에요? You can say all of them. (1), (2) is most usually used in a daily life. These two means the same, cause only the word '저녁' itself means '저녁 밥(dinner)'. (3) 저녁 '식사' is more polite form of 저녁 '밥', but most situation 저녁 '밥' can be used, cause '밥' is also not an informal word. I hope this would be helpful :)