Concept:Animal by-products such as horn are hard, durable, and easy to shape, so they are used to make daily items like buttons.
Explanation:The question asks which animal product is used in button production.
Skin is mainly used for making leather goods.
Fur is used for clothing and decoration.
Bone can be carved, but it is not the usual material for buttons.
Horn is strong and can be easily cut or polished into small button shapes.
Therefore, horn is the correct animal product used for making buttons.
Answer:D. Horn