Which of the following associations of an animal germ layer is correct?

The association of mesoderm with the formation of muscles and most internal organs is correct because the mesoderm is one of the three primary germ layers in embryonic development. It lies between the ectoderm, which develops into structures such as the skin and nervous system, and the endoderm, which forms internal structures like the lining of the digestive tract and respiratory system.

During embryogenesis, the mesoderm gives rise to several critical components of the body, including skeletal muscle, smooth muscle, cardiac muscle, connective tissues, the heart, blood vessels, kidneys, and parts of the reproductive organs. This layer plays a vital role in developing the body's framework and internal systems, making it essential for the proper functioning and structure of an organism.

The other associations listed result in the formation of incorrect structures; the ectoderm is incorrect for the lining of the digestive tract, as that is primarily derived from the endoderm. The central nervous system also derives from the ectoderm and not the mesoderm, while the endoderm does not form the outer covering of an organism. This clarification highlights the specific and essential developmental roles of each germ layer in forming various tissues and organs in animals.