Final answer:
Dense concentrations of muscle spindles are found in skeletal muscles, which are under voluntary control. Smooth muscles lack striations and are involuntary, and cardiac muscles have unique features like intercalated discs but are also involuntary.
Step-by-step explanation:
The correct answer to the question of which skeletal muscles contain dense concentrations of muscle spindles is c) Skeletal muscles. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via sensory neurons. This system is also responsible for the stretch reflex. Skeletal muscles are under voluntary control, allowing us to consciously move different parts of our body.
Smooth muscles, such as those found in the walls of the intestines and blood vessels, differ from skeletal muscles because they lack striations. Additionally, cardiac muscles, which are found only in the heart, have unique features such as intercalated discs and also operate involuntarily. Involuntary muscles include both smooth and cardiac muscles and cannot be controlled by an individual's will.