Which of the following best describes a rabbit classified as an adult?

A rabbit classified as an adult is best described as a fully developed rabbit of breeding age. This classification typically refers to rabbits that have reached sexual maturity and are capable of reproduction, which generally occurs around 6 months of age for most domestic breeds. At this stage, the rabbit has achieved its full physical development and is considered mature, both physically and behaviorally.

The other choices do not accurately define an adult rabbit. For instance, younger rabbits are considered juvenile or growing and may be less than four months old or just ready to be weaned, indicating they are still developing and not fully mature. Similarly, the option that mentions a rabbit that has not reached its full size encompasses those juvenile rabbits, as they have not attained their full physical potential or breeding status. Therefore, the best definition of an adult rabbit is one that has reached breeding age and is fully developed.