Which of the following is a primary action of the quadriceps muscle group?

The quadriceps muscle group is primarily responsible for knee extension. This group consists of four muscles: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. When these muscles contract, they work together to straighten the knee joint, which is essential for activities such as standing up, walking, running, and jumping.

While the other options involve movements at the knee or hip, they do not accurately represent the primary function of the quadriceps. Knee flexion, for example, is the opposite action and primarily involves the hamstrings. Hip adduction pertains to muscles that bring the leg toward the body's midline, a function that is unrelated to the quadriceps. Lastly, hip extension is primarily performed by the gluteus maximus and hamstrings rather than the quadriceps. Thus, understanding the anatomy and function of the quadriceps clarifies why knee extension is their primary action.