The correct answer is sartorius. The sartorius muscle is a long, thin muscle that runs down the length of the thigh. It is the longest muscle in the human body and is responsible for flexing, abducting, and laterally rotating the hip joint. It also helps to flex the knee joint. The other muscles listed in the options, such as the quadriceps femoris and rectus femoris, are also muscles in the thigh, but they have different functions and are not the correct answer in this case.