The statement that indicates the need for further instruction is "C) I can apply powder under the liner to help with sweating."
The halo vest is used to immobilize the cervical spine after a spinal cord injury, and the pins of the halo device are secured to the skull. Applying powder under the liner can increase the risk of infection and looseness of the pins, which can lead to complications such as movement of the halo. Therefore, the nurse should instruct the client to avoid applying powder under the liner.
The other statements made by the client are correct. Changing the vest liner periodically and checking for blisters and redness are important measures to prevent skin breakdown and infection. If a pin becomes detached, it is important to notify the surgeon immediately as this can compromise the effectiveness of the halo device.