Final answer:
Tie-downs must have a minimum combined strength that is capable of lifting twice the weight of the cargo to ensure safety and stability during transport, which corresponds to option c. This principle ensures that the tie-downs can handle forces that might be encountered during normal conditions of transport.
Step-by-step explanation:
The student's question pertains to the strength requirements for cargo tie-downs in the context of physics and engineering principles. These strength requirements ensure safety and prevent the cargo from shifting or falling during transport. Although the provided reference text does not directly answer the question, it does briefly touch upon concepts of forces and buoyancy, which can be related to the principles of securing a load.
The reference text states that a ship can carry a load up to nine times its weight without sinking, thanks to the maximum buoyant force being ten times the weight of the ship's steel. This emphasizes that the load-bearing capacity is dependent on the strength and the physical properties of the material involved. Similarly, tie-downs must have a combined strength capable of handling certain multiplications of the cargo weight they secure.
In this scenario, considering standard safety guidelines in transportation and engineering, tie-downs should be designed to handle forces greater than the actual weight of the cargo. The correct option in the final answer that describes the minimum combined strength of cargo tie-downs required to lift the cargo safely is c. Twice the weight of the cargo tied down.