Final answer:
A CT scan is the type of test used to show damage in closed-head injuries, as it can construct images that reveal tumors or brain atrophy.
Step-by-step explanation:
The type of test to perform on closed-head injuries to show damage is b. CT scan (Computed Tomography). A CT scan involves taking several X-rays of a particular section of a person's body or brain. These X-rays pass through tissues of different densities at varying rates, which allows a computer to construct an image of the area being scanned. This imaging technique is often used to determine the presence of issues such as tumors or brain atrophy, making it a valuable tool for detecting brain damage resulting from head injuries.