Final answer:
The iliopsoas complex is the major flexor of the hip.
Step-by-step explanation:
The answer to this question is option D: The iliopsoas complex is the major flexor of the hip. This statement is true. The iliopsoas complex consists of two muscles, the iliacus and the psoas major, which work together to flex the thigh at the hip joint. These muscles are responsible for bringing the thigh forward towards the chest. Examples of activities that involve hip flexion are walking, running, and cycling.
Option A is incorrect because two-joint muscles actually function more effectively at both joints when the position of the other joint stretches the muscle slightly. This allows the muscle to work across both joints simultaneously. Option B is also incorrect because option D is true. Option C is incorrect because it contradicts the correct statement in option D.