**Core Concept**
Human locomotion involves a complex interplay of muscle forces and joint movements to maintain posture and stability. In walking, the trunk and pelvis are subject to unwanted tilting due to gravity, which can disrupt balance and gait. To prevent this, the body employs specific mechanisms to counteract gravitational forces.

**Why the Correct Answer is Right**
The major factor in preventing unwanted movement of the pelvis and trunk during walking is the contraction of the abdominal muscles, particularly the transversus abdominis muscle. This muscle wraps around the abdominal cavity and helps to stabilize the lumbar spine and pelvis by increasing intra-abdominal pressure. The increased pressure compresses the vertebrae and pelvis, preventing them from tilting to the unsupported side. This mechanism is essential for maintaining balance, stability, and efficient locomotion.

**Why Each Wrong Option is Incorrect**
**Option A:** The gluteus maximus muscle plays a crucial role in hip extension and external rotation, but it is not the primary factor in preventing unwanted movement of the pelvis and trunk.
**Option B:** The iliopsoas muscle contributes to hip flexion and lumbar lordosis, but it does not directly counteract gravitational forces on the pelvis and trunk.
**Option C:** The latissimus dorsi muscle is involved in shoulder joint movement and scapular rotation, but it is not directly related to pelvic and trunk stability during walking.

**Clinical Pearl / High-Yield Fact**
The transversus abdominis muscle is often referred to as the "core" muscle of the abdominal wall, and its contraction is essential for maintaining spinal stability and preventing back pain.

**Correct Answer: C. The contraction of the abdominal muscles, particularly the transversus abdominis muscle.**