Which of the following must be true for a second in command to operate an aircraft in domestic operations?

For a second in command (SIC) to operate an aircraft in domestic operations, it is essential for that individual to hold a commercial certificate with the appropriate ratings. This requirement ensures that the SIC has the necessary knowledge, skills, and qualifications to contribute effectively and safely to the flight operations.

Holding a commercial certificate signifies that the pilot has achieved a certain level of proficiency and is qualified to engage in paid flying, which is critical in a commercial aviation environment. Additionally, the proper ratings associated with the certificate ensure that the SIC is endorsed to operate the specific aircraft type being flown, further enhancing the safety and efficacy of the flight.

Understanding the responsibilities of a SIC also ties back to regulatory requirements and operating standards that govern flight safety. Pilots without the necessary certifications and ratings may lack the essential training required for the role, potentially jeopardizing flight safety.

In contrast, while having a first-class medical and a type rating for all aircraft would be beneficial, they are not universally required for the role of SIC in domestic operations. Similarly, while experience in another aircraft type may enhance a pilot's skills, it is not a mandatory requirement for the SIC position.