What constitutes a significant part of milk solids sold as required by Federal Milk Market Orders?

The correct choice focuses on milk fat and protein content, which are crucial components of milk solids as defined by Federal Milk Marketing Orders. These components not only are significant nutritional elements but also play a critical role in determining the quality and grade of milk. Milk fat contributes to the creaminess and mouthfeel of dairy products, while protein is essential for various functional properties such as emulsification and texture.

In the context of pricing and market orders, milk fat and protein levels are often used to establish payment rates for dairy farmers, reflecting their product's value in the marketplace. This makes milk fat and protein content vital for compliance with regulatory standards that dictate how much of these solids must be present for the milk to be classified for certain markets.

The other choices focus on components that, while they play roles in milk processing, do not constitute the primary standards for milk solids in the same way that fat and protein do. Flavors, colors, milk sugar, water, salt, and preservatives may influence a milk product's characteristics or shelf life but do not have the same regulatory weight or market significance as fat and protein.