Heredity affects flexibility in several ways, particularly in how it influences joint structure, muscle composition, and overall body mechanics. Therefore, the most accurate option would be:
C. Heredity affects the amount of flexibility present in joints.
This option captures the essence of how genetic factors can determine the natural range of motion and flexibility that an individual may possess. However, option D could also be considered correct as it touches on the structural aspects that heredity might influence. If you have to choose one, C would be the best fit regarding direct effects on flexibility.