Fire detecting systems on merchant vessels may be arranged to sense __________.

Correct Answer: Option D — all of the above

Modern marine fire detection systems utilize diverse sensing technologies to ensure rapid response. Ionization sensors detect combustion particles, smoke detectors identify particulate matter, and rate-of-rise sensors monitor rapid thermal increases. Integrating these technologies provides comprehensive coverage for various fire types, including smoldering fires and high-energy fuel-based combustion events.