Final answer:
The correct analogy for 'ribosomes are to the RER' is 'tires are to a car', highlighting the importance of ribosomes for protein synthesis in the rough endoplasmic reticulum.
Step-by-step explanation:
The analogy that best completes the statement 'ribosomes are to the RER (rough endoplasmic reticulum)' is b. tires are to a car. This analogy is appropriate because ribosomes are essential components that are attached to the rough endoplasmic reticulum, much like tires are essential components that are attached to a car. Without tires, a car would not be able to function effectively, and similarly, without ribosomes, the RER would not be able to function in its role of protein synthesis. Ribosomes are cellular substructures where proteins are synthesized, and they are directly involved in producing the proteins that are crucial for cell function.