Final answer:
An 2) open fracture, characterized by a broken bone causing an external wound, is the correct answer to the student's question. This type of fracture increases the risk of infection due to the skin being penetrated.
Step-by-step explanation:
The type of fracture that involves a broken bone piercing through the skin causing an external wound is known as an open fracture. This kind of fracture is severe because it creates an open wound, which exposes the bone and the internal tissue, increasing the risk of infection and requiring immediate medical attention. An open fracture is different from a closed fracture, where the skin remains intact.
Other types of fractures include comminuted, where the bone breaks into several pieces, impacted where one fragment of bone goes into another, and greenstick, common in children where the bone bends and cracks instead of breaking completely. A pathological fracture occurs when an underlying disease weakens the bone. Thus, among the presented options, the correct answer is 2) Open.