The appropriate base for soup preparation is stock.
Stock is a flavorful liquid that is made by simmering bones, meat, vegetables, and aromatics (such as onions, carrots, and celery) in water for an extended period of time. This slow cooking process extracts the nutrients, flavors, and gelatin from the ingredients, resulting in a rich, flavorful liquid that can be used as the base for soups, stews, sauces, and more.
Salsa, corn flour, and white roux are not appropriate bases for soup preparation. Salsa is a type of sauce made from tomatoes, onions, chilies, and other ingredients, and is typically used as a condiment for tacos, chips, and other dishes. Corn flour is a fine powder made from ground corn, and is often used as a thickener for sauces, stews, and other dishes. White roux is a mixture of flour and fat (such as butter or oil) that is cooked over low heat to form a paste-like substance, which is used as a thickener for soups, sauces, and other dishes.
In summary, stock is the appropriate base for soup preparation because it is a flavorful liquid that is made by simmering bones, meat, vegetables, and aromatics in water for an extended period of time, resulting in a rich, flavorful liquid that can be used as the base for soups, stews, sauces, and more.