The correct answer is option 2: Milk. Milk is a rich source of Vitamin B-12. Vitamin B-12 is an essential nutrient that plays a crucial role in the production of red blood cells and the maintenance of the nervous system. It is particularly important for vegetarians and vegans as it is primarily found in animal-based foods. Cashew nuts, apples, and watermelon are not significant sources of Vitamin B-12. Cashew nuts are a good source of other nutrients like healthy fats and minerals, but they do not contain Vitamin B-12. Similarly, apples and watermelon are fruits that mainly provide vitamins, fiber, and antioxidants but do not supply a significant amount of Vitamin B-12.