**Core Concept**
The contraction of the diaphragm is a critical process in the respiratory system, allowing for the expansion of the thoracic cavity and the subsequent inhalation of air. This process is primarily facilitated by the diaphragm's unique structure and its innervation.

**Why the Correct Answer is Right**
During contraction, the diaphragm flattens, increasing the volume of the thoracic cavity. This is achieved through the contraction of the diaphragm's muscle fibers, which are primarily innervated by the phrenic nerves that emerge from the spinal cord at the level of the cervical vertebrae (C3-C5). As the diaphragm flattens, its dome shape decreases, allowing the lungs to expand and fill with air. This process is essential for normal breathing and is a critical component of the respiratory system's function.

**Why Each Wrong Option is Incorrect**
**Option A:** The nerves responsible for the contraction of the diaphragm, the phrenic nerves, emerge from the spinal cord at the level of the cervical vertebrae (C3-C5), not the lower thorax.
**Option C:** During contraction, the diaphragm actually increases the lateral distance between the lower rib margins, not reduces it. This is because the diaphragm's contraction pushes the rib cage upward and outward, increasing the volume of the thoracic cavity.
**Option D:** The contraction of the diaphragm causes the abdominal wall to move outward, not inward. This is because the diaphragm's contraction pushes the abdominal contents upward and outward, increasing the intra-abdominal pressure.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the diaphragm's contraction is a critical component of the respiratory system's function, and its abnormal contraction or relaxation can lead to respiratory problems, such as respiratory failure or asthma.

**✓ Correct Answer: B. It tends to flatten the diaphragm**