You didn't really provide any possible choices here but when we're talking about VO2max, we're talking about t emaximal oxygen consumption. Sometimes it's also refered to as our maximal aerobic capacity.
VO2max is the maximum amount of how much oxygen we can consume during a certain exercise. In clinical and medical settings treadmills, where the difficulty gets incread, are usually used. VO2max is considered to reflect our aerobic physical capabilities.
