Which of these is not a fruit?

Onion is not a fruit because it is a vegetable. Unlike fruits, onions do not develop from the ovary of a flowering plant and contain seeds. Instead, onions are bulbs that grow underground and are used as a culinary ingredient for their savory flavor. While apples, oranges, and mangoes are all fruits that develop from the flowering plants' ovaries and contain seeds, onions are classified as vegetables due to their different botanical characteristics.