Concept:Millon's reagent is used to test for the presence of proteins.Solution:A red precipitate with Millon's reagent indicates protein.Groundnut (peanut) is rich in protein.Banana, orange juice, and pineapple juice are not primarily protein foods.Key point: Millon's test is a protein test; for starch use iodine solution and for reducing sugars use Benedict's or Fehling's solution.Answer:Option B (Groundnut)