Analogy Explanation:

In this analogy, the relationship between "rung" and "ladder" is being compared to the relationship between "vertebrae" and an option given in the question. We need to identify the option that represents a similar relationship to the one between rung and ladder.

Rung and Ladder relationship:
A ladder is made up of several rungs, which are the horizontal steps that one can climb on. Without rungs, a ladder would not be able to serve its purpose of providing steps for climbing.

Vertebrae and Backbone relationship:
The correct option in this analogy is "c) Backbone". The backbone is made up of several vertebrae, which are the individual bones that make up the spinal column. Without vertebrae, the backbone would not be able to provide support and protection to the spinal cord.

Incorrect Options:

a) Axle: An axle is a shaft that rotates with the wheels of a vehicle. Axles are not directly related to vertebrae or the backbone, so this option is not the correct analogy.

b) Plankton: Plankton refers to a diverse group of small organisms that float or drift in water. Plankton is not related to vertebrae or the backbone, so this option is not the correct analogy.

d) Wheel: A wheel is a circular object that rotates on an axle to facilitate movement. Wheels are not directly related to vertebrae or the backbone, so this option is not the correct analogy.

Conclusion:

The correct option for the analogy "rung is to ladder as vertebrae is to" is "c) Backbone". The relationship between rung and ladder is similar to the relationship between vertebrae and the backbone, as both involve individual components making up a larger structure.