## **Core Concept**
The question pertains to the muscles involved in respiration, specifically focusing on their activity during normal, quiet breathing. Normal respiration primarily involves the diaphragm and other accessory muscles to a lesser extent.

## **Why the Correct Answer is Right**
The correct answer, , refers to the accessory muscles of respiration, which include sternocleidomastoid, scalene, and abdominal muscles. During normal, quiet breathing, these muscles are not actively engaged. The primary muscle for inspiration is the **diaphragm**, which contracts and moves downward, increasing the volume of the thoracic cavity. Expiration during quiet breathing is largely **passive**, resulting from the elastic recoil of the lungs and the thoracic cavity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the diaphragm is actively involved during normal respiration, being the primary muscle for inspiration.
- **Option B:** This option is incorrect as it suggests another set of muscles, but without specifying, it's hard to directly refute; however, given that accessory muscles are indeed inactive during normal breathing, an option suggesting active muscles during normal breathing would be incorrect.
- **Option D:** This option is incorrect because it implies that another group of muscles or a specific condition related to respiration is active or relevant, which does not align with the context of muscles inactive during normal breathing.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that during **forced expiration**, such as in exercise or coughing, accessory muscles like the **abdominal muscles** and **internal intercostal muscles** become active. In contrast, during **normal, quiet breathing**, these muscles remain inactive, and the process is primarily driven by the diaphragm.

## **Correct Answer: C.**