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Lifelines on a ship are crucial for enhancing safety by preventing accidents overboard. They are typically strong ropes or cables stretched along the edges of decks to provide a barrier that helps keep crew members and guests safely onboard, especially in rough seas or during unusual movements of the vessel. By acting as a protective measure, lifelines significantly reduce the risk of individuals falling into the water, which can lead to serious accidents or fatalities.

Regarding the other options, enhancing shipboard communications primarily involves radios and other electronic devices, not lifelines. Improving cargo loading efficiency pertains more to structural designs and equipment used in handling cargo rather than safety features. Monitoring weather conditions is typically the function of specific instruments and personnel trained for that purpose, rather than lifelines themselves. Thus, the primary function of lifelines is centered around preventing individuals from going overboard, making that the correct and most relevant answer.