Which casualty involving a mobile offshore drilling unit would require a report to be filed?

Correct Answer: Option A — An occurrence materially and adversely affecting the vessel's fitness for service

46 CFR 4.05-1 defines reportable marine casualties to include occurrences that materially and adversely affect a vessel's seaworthiness or fitness for service. This broad requirement ensures the Coast Guard is informed of any structural or mechanical failure that could compromise the safety of the unit or its crew.