**Core Concept**
The question is testing the knowledge of the major muscles involved in hip joint movement, specifically the flexion of the hip joint. The hip joint is a ball-and-socket joint that allows for a wide range of movements, including flexion, extension, abduction, adduction, and rotation.

**Why the Correct Answer is Right**
The major flexor of the hip joint is the iliopsoas muscle, which is a powerful muscle that plays a crucial role in hip flexion. The iliopsoas muscle is formed by the convergence of the iliacus and psoas major muscles. The psoas major muscle originates from the lumbar vertebrae and inserts into the lesser trochanter of the femur, while the iliacus muscle originates from the iliac fossa and inserts into the same point. The iliopsoas muscle contracts to flex the hip joint, bringing the thigh towards the abdomen.

**Why Each Wrong Option is Incorrect**
**Option A:** The gluteus maximus muscle is actually the major extensor of the hip joint, not the flexor. It contracts to extend the hip joint, which is the opposite of flexion.
**Option B:** The tensor fasciae latae muscle assists in hip flexion, but it is not the major flexor of the hip joint. It contracts to assist in hip flexion and abduction.
**Option C:** The adductor magnus muscle is involved in hip adduction, not flexion. It contracts to bring the thigh towards the midline of the body.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the iliopsoas muscle is a major flexor of the hip joint, and its contraction is responsible for bringing the thigh towards the abdomen. This muscle is often involved in hip flexion contractures, which can occur in conditions such as cerebral palsy or spinal cord injury.

**Correct Answer:** D. Iliopsoas muscle.