Final answer:
Portable radios are a common means of communication for crew members on vessels.
Step-by-step explanation:
The true statement about portable radio operation onboard a vessel is option C: Portable radios are a common means of communication for crew members on vessels. Portable radios are used for communication purposes on vessels, allowing the crew members to stay connected and coordinate their activities. They can be used for important safety communication, as well as operational and logistical communication.