Which of the following fractures involves the piercing of bone through the skin?

A fracture that involves the piercing of bone through the skin is specifically referred to as a compound fracture, also known as an open fracture. This type of fracture not only involves a break in the bone but also results in an open wound that connects the fractured bone to the outside environment. This scenario increases the risk of infection, as bacteria can enter through the open skin.

In contrast, other types of fractures, such as simple fractures, occur without any external wounds, while greenstick fractures are typically incomplete fractures common in children. Comminuted fractures involve the bone being shattered into multiple pieces but do not necessarily imply that the skin is broken. Therefore, the defining characteristic of a compound fracture is the communication between the fractured bone and the external environment, making it the correct answer in this context.