Final answer:
Rolled oats are not significant sources of vitamin D, unlike salmon, sardines, fortified milk, and certain mushrooms. The skin can synthesize vitamin D with sunlight exposure and it's often added to food products like milk and cereal in the U.S.
Step-by-step explanation:
The significant food sources of vitamin D notably include fatty fish such as salmon and sardines, fortified milk, and even certain types of mushrooms that have been exposed to ultraviolet light. However, rolled oats do not naturally contain vitamin D or are typically fortified with this nutrient, making them the correct answer to which food source listed is least significant for vitamin D.
Vitamin D can also be synthesized by the skin when exposed to sunlight, and is necessary for maintaining healthy bone calcium levels. Foods such as cod liver oil, fish liver oil, and egg yolk are rich dietary sources. In fortified foods, the United States often adds vitamin D to milk, soy milk, and breakfast cereals to ensure adequate intake for those who may not get enough sunlight, typically providing specific amounts of vitamin D per serving.