**Core Concept**
The question requires identification of the primary muscles responsible for the process of expiration in the respiratory system. Expiration is the phase of breathing where the lungs release air, and it is primarily facilitated by the contraction of specific muscle groups.

**Why the Correct Answer is Right**
The correct answer involves the internal intercostal muscles and the abdominal muscles, particularly the rectus abdominis. During expiration, the internal intercostal muscles contract, which pulls the ribs downward and inward, increasing the pressure inside the thoracic cavity and forcing air out of the lungs. Additionally, the contraction of the abdominal muscles, including the rectus abdominis, increases intra-abdominal pressure, which also contributes to the compression of the lungs and the expulsion of air. This process is crucial for maintaining proper respiratory function and is essential for patients with respiratory conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** The diaphragm is actually the primary muscle responsible for inspiration, not expiration. During inspiration, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and allowing air to enter the lungs.

**Option C:** The external intercostal muscles are involved in inspiration, not expiration. During inspiration, the external intercostal muscles contract, which pulls the ribs upward and outward, increasing the volume of the thoracic cavity and allowing air to enter the lungs.

**Option D:** The rectus abdominis is indeed involved in expiration, as explained earlier. However, the diaphragm is not, making option D incorrect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the respiratory muscles work in a coordinated manner to maintain proper breathing. During expiration, the internal intercostal muscles and abdominal muscles, particularly the rectus abdominis, contract to increase pressure inside the thoracic cavity and force air out of the lungs. This coordination is essential for patients with respiratory conditions, such as chronic obstructive pulmonary disease (COPD), where maintaining proper breathing mechanics is critical.

**✓ Correct Answer: B. bd. Internal intercostal and Rectus Abdominis muscles are involved in expiration.**