Correct Option: D

Heart is the only organ in the body which never rest throughout the entire life. The heart is a hollow muscle that pumps blood throughout the blood vessels by repeated, rhythmic contractions. It is found in all animals with a circulatory system (including all vertebrates). The vertebrate heart is principally composed of cardiac muscle and connective tissue. The average human heart, beating at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan.