I need to find out what an incomplete protein is, I have the options of a casserole made with quinoa, a handful of peanuts, walnuts, and cashews, a...

I need to find out what an incomplete protein is, I have the options of a casserole made with quinoa, a handful of peanuts, walnuts, and cashews, a sandwich made with turkey, or a cup of soy milk. How do I know which single food item is a source of incomplete protein? (only one)