Which peripheral adaptation is in part responsible for improvements in oxygen consumption that result from regular cardiovascular exercise?

Increased stroke volume is a central cardiac adaptation, not a peripheral one, and the question specifically asks about peripheral adaptations.

Endurance training increases mitochondrial density (not decreases it), which is another key peripheral adaptation supporting greater oxidative capacity.

Regular endurance exercise stimulates angiogenesis, increasing the number of capillaries surrounding each muscle fiber. This reduces diffusion distance for oxygen, increases the surface area for gas exchange, and prolongs red blood cell transit time through the muscle, all of which enhance peripheral oxygen extraction and raise maximal oxygen consumption.

Venous return increases with training due to improved muscle pump and plasma volume expansion; decreased venous return would impair, not improve, oxygen delivery and cardiac output.