Concept:A by-product is a secondary item obtained after the main product, such as meat, has been utilized.
Explanation:Livestock includes animals like cattle, sheep, goats, and pigs.
Hides and skins are obtained from these animals after slaughter.
This makes hide a common by-product across all such livestock.
Meat is the primary product, not a by-product.
Milk is produced only by milking animals, not by all livestock.
Horns are present only in some breeds, not all.
Therefore, hide is the correct by-product common to all livestock.
Answer:A. hide