Which of the following factors can significantly influence individual fitness levels?

Genetics plays a crucial role in determining individual fitness levels, as it influences various physiological traits that can affect performance and health. Factors such as muscle fiber composition, metabolism, and cardiovascular efficiency are all determined to some extent by an individual’s genetic makeup. For example, some individuals may have a genetic predisposition toward having a higher percentage of fast-twitch muscle fibers, making them more suited for explosive activities like sprinting, while others may have more slow-twitch muscle fibers, lending themselves better to endurance activities.

While socioeconomic status can impact access to fitness resources and knowledge, and the time of day can affect energy levels and performance due to circadian rhythms, these factors do not have the inherent biological influence that genetics does. Additionally, while the type of gym membership might offer different facilities or environments for exercise, it doesn't fundamentally alter an individual's biological capabilities or fitness potential. Thus, genetics is a primary determinant of fitness levels and is a significant factor in understanding individual differences in physical ability.