Heredity is best described as:

Heredity refers to the genetic traits and characteristics that are passed down from parents to their offspring. When discussing the context of fitness, heredity can play a significant role in determining a person's natural abilities, such as muscle characteristics, metabolic rate, and overall physical potential. These traits are established at birth and can influence how an individual responds to training and their overall capacity for physical performance.

While individuals may improve their fitness through training, nutrition, and lifestyle changes, the hereditary factors themselves are largely immutable. This means that a person's genetic makeup provides a foundation that cannot be altered. For example, some individuals may have a genetic predisposition for being naturally strong or having high endurance, while others may not have the same genetic advantages. Thus, recognizing heredity as natural abilities that cannot be changed acknowledges the underlying genetic influence that shapes fitness potential.

In the context of the other options, while factors like training and environment are influential, they interact with the genetic predispositions, which is why acknowledging the fixed nature of hereditary traits provides a clearer understanding of fitness development.