When logging into the Inmarsat system using Inmarsat-C, it is necessary to:

Correct Answer: Option C — Select the Ocean Region.

Inmarsat-C systems require the operator to manually select the appropriate Ocean Region (IOR, AOR-E, AOR-W, or POR) based on the vessel's geographic position. This selection directs the terminal to tune its antenna and frequency to the specific satellite serving that region, which is a prerequisite for successful network registration.