Which of the following is a common by-product of farm animals?

All of the above (wool, fertilizer, and milk) are common by-products of farm animals.

Wool is a by-product of sheep farming. It is the soft, curly hair that grows on the sheep's body. Wool is commonly used to make warm clothing and fabrics.

Fertilizer is a by-product of animal farming. It is produced from the manure (feces) of farm animals like cows, pigs, and chickens. Manure contains important nutrients like nitrogen, phosphorus, and potassium, which are essential for plant growth. Farmers use animal manure as a natural fertilizer to improve soil fertility and promote healthy plant growth.

Milk is another common by-product of farm animals, especially cows, goats, and sheep. Milk is produced by these animals to feed their young. Humans also use milk as a nutritious source of food. It is rich in essential nutrients like calcium, protein, and vitamins.

Therefore, all of the options mentioned (wool, fertilizer, and milk) are common by-products of farm animals.