What is lactic acid derived from?

Lactic acid is primarily derived from sour milk, which undergoes fermentation by lactic acid bacteria, resulting in lactic acid production. This process is commonly observed in dairy products such as yogurt and certain cheeses, where the fermentation of lactose (the sugar found in milk) generates lactic acid as a byproduct.

Sour milk contains high levels of lactic acid due to the activity of bacteria that thrive in dairy environments, leading to the characteristic sour taste and thickening of the milk. While tomatoes and other plants don't typically produce lactic acid through fermentation, they can be involved in various fermentation processes that might produce other compounds.

In this context, options that reference fruits and berries, vegetables and greens, or grains and cereals do not align with the primary sources of lactic acid, which are strongly rooted in dairy fermentation. Therefore, the choice indicating sour milk as the source of lactic acid is the most accurate.