**Core Concept**
Composite muscles are muscles that contain more than one type of fiber, such as skeletal muscle and smooth muscle, or different types of skeletal muscle fibers. This unique composition allows them to perform a wide range of functions, including involuntary contractions and voluntary movements. Examples of composite muscles include the diaphragm, which contains skeletal muscle and smooth muscle.

**Why the Correct Answer is Right**
The correct answer is a muscle that is composed of a single type of fiber, such as skeletal muscle or smooth muscle. The diaphragm, for instance, is a composite muscle that contains both skeletal muscle and smooth muscle fibers, allowing it to perform both voluntary and involuntary contractions.

**Why Each Wrong Option is Incorrect**
**Option A:** The rectus abdominis is a composite muscle that contains both skeletal muscle and smooth muscle fibers, making it a correct example of a composite muscle.
**Option B:** The psoas major muscle is a composite muscle that contains both skeletal muscle and smooth muscle fibers, making it a correct example of a composite muscle.
**Option C:** The sternocleidomastoid muscle is a composite muscle that contains both skeletal muscle fibers, making it a correct example of a composite muscle.

**Clinical Pearl / High-Yield Fact**
One key characteristic of composite muscles is their ability to perform a wide range of functions, including involuntary contractions and voluntary movements. This is due to the unique composition of these muscles, which allows them to respond to different types of stimuli.

**Correct Answer:** D.