Lactic acid is primarily derived from which of the following substances?

Lactic acid is primarily derived from sour milk, which is a result of the fermentation process where lactose is converted into lactic acid by lactic acid bacteria. This process is essential in the production of various dairy products, such as yogurt and certain cheeses, making sour milk a primary source of lactic acid.

In this context, the fermentation of milk not only contributes to the tangy flavor associated with sour milk but also enhances its nutritional profile through the presence of probiotics. The other substances listed—citrus fruits, almonds, and apples—are not known for significant lactic acid production. Instead, these foods may contain other types of organic acids, like citric acid in citrus fruits, but do not serve as a primary source of lactic acid. Thus, sour milk is the correct choice, grounded in its well-known association with lactic acid fermentation.