Final answer:
The depth of a tunneled wound should be accurately measured with a sterile, flexible applicator moistened with saline or a sterile cotton swab for precision and safety.
Step-by-step explanation:
To measure the depth of a patient's tunneled wound accurately, the appropriate tool to use is a sterile, flexible applicator moistened with saline. This method allows for a gentle and precise measurement of the wound's depth. A sterile cotton swab could also be suitable if a flexible applicator is not available. Rulers or measuring tapes would be inappropriate as they are not sterile and could harm the wound, while pen and paper for estimation would not provide the precise measurement needed for medical documentation and treatment planning.