The correct answer is option 'C' - Cardiac muscles.

Cardiac muscles, also known as myocardium, are a unique type of muscle found exclusively in the heart. They differ from both voluntary or skeletal muscles and involuntary or smooth muscles in terms of structure and function.

Structure of Cardiac Muscles:
- Cardiac muscles are striated muscles, meaning they have a striped appearance under a microscope.
- They are made up of individual cells called cardiomyocytes that are connected by intercalated discs.
- Intercalated discs contain specialized protein structures called gap junctions and desmosomes, which allow for coordinated contraction of cardiac muscle cells.
- Cardiac muscle cells are branched and connected in a three-dimensional network, forming a complex meshwork that allows for efficient transmission of electrical signals.

Function of Cardiac Muscles:
- The primary function of cardiac muscles is to contract and relax rhythmically to pump blood throughout the body.
- Unlike voluntary muscles, cardiac muscles contract involuntarily and continuously, without conscious control.
- The contraction and relaxation of cardiac muscles are regulated by electrical signals generated within the heart itself.
- These electrical signals are initiated by the sinoatrial (SA) node, also known as the natural pacemaker of the heart, and are conducted through the atria, atrioventricular (AV) node, and specialized conducting fibers to ensure synchronized contractions of the heart chambers.
- The coordinated contraction of cardiac muscles allows for efficient pumping of blood, ensuring a constant supply of oxygen and nutrients to the body's organs and tissues.

Differences from Skeletal and Smooth Muscles:
- Skeletal muscles are attached to bones and are responsible for voluntary movements, such as walking or lifting objects.
- Skeletal muscles are also striated, but their cells are long and cylindrical, with multiple nuclei.
- Smooth muscles, on the other hand, are found in the walls of hollow organs, blood vessels, and other structures.
- Smooth muscles are responsible for involuntary movements, such as the contraction of the digestive tract or blood vessels.
- Smooth muscles lack striations and have spindle-shaped cells with a single nucleus.

In summary, cardiac muscles are a specialized type of striated muscle found only in the heart. They contract involuntarily and rhythmically to pump blood throughout the body, ensuring the proper functioning of the cardiovascular system. Unlike skeletal and smooth muscles, cardiac muscles have unique structural and functional characteristics that enable their specific role in the heart.