Final answer:
Spinach salad is the richest source of vitamin K among the options given. Spinach and other green leafy vegetables are known for their high vitamin K content, which is not substantially found in enriched breads, roast chicken or citrus fruits.
Step-by-step explanation:
The question regarding which food is a rich source of vitamin K relates directly to nutritional content of different food groups. Among the options provided, spinach salad is the richest source of vitamin K. Spinach, along with other green leafy vegetables, is well-known for its high vitamin K content. Vitamin K is essential for blood clotting and bone health among other biological roles. However, foods like enriched breads, roast chicken, and citrus fruits do not provide substantial amounts of vitamin K compared to green leafy vegetables.
Animal products generally contain less vitamin K, with the exception of certain products like milk and eggs which have small quantities. Vitamin K2 is also produced by bacteria in the human intestine. It's important to include a balanced diet rich in green leafy vegetables to ensure adequate intake of vitamin K.