Final answer:
The National Transportation Safety Board (NTSB) investigates airliner crashes and determines their probable cause, separate from the FAA, NASA, or ICAO.
Step-by-step explanation:
The agency responsible for investigating airliner crashes and determining their 'probable cause' is C. National Transportation Safety Board (NTSB). The NTSB is an independent federal agency charged with investigating civil aviation accidents in the United States and issuing safety recommendations aimed at preventing future accidents.
Although the Federal Aviation Administration (FAA) regulates and oversees all aspects of U.S. civil aviation, the NTSB is the agency that steps in when something goes wrong, such as an airline accident. It should not be confused with NASA, which focuses primarily on space exploration and aeronautics research, or the ICAO, which is a specialized agency of the United Nations charged with coordinating international air travel and establishing universal aviation standards and practices.