The correct answer is: C. Bacteria
Bacteria are single-celled organisms that are found in almost every environment on Earth. They are essential for the production of dairy products such as butter, cheese, and yogurt.
Bacteria are used to ferment milk, which is the process of converting milk into a solid or semi-solid form. This is done by adding bacteria to milk and allowing them to multiply. The bacteria produce lactic acid, which causes the milk to curdle. The curds are then separated from the whey and pressed into the desired shape.
Different types of bacteria are used to produce different types of dairy products. For example, Lactobacillus bulgaricus and Streptococcus thermophilus are used to make yogurt, while Lactobacillus casei and Propionibacterium freudenreichii are used to make Swiss cheese.
Bacteria are also used to produce other fermented foods, such as sauerkraut, kimchi, and tempeh.
Viruses are not living organisms and cannot be used to produce dairy products. Fungi are multicellular organisms that can be used to produce some fermented foods, such as tempeh and miso. Algae are single-celled organisms that are found in water. They are not used to produce dairy products.