**Core Concept**
Single unit smooth muscles are a type of smooth muscle tissue that contracts as a single unit, meaning that all the muscle cells are electrically coupled and contract together. This type of smooth muscle is often found in the walls of hollow organs such as the gastrointestinal tract, blood vessels, and airways.

**Why the Correct Answer is Right**
Single unit smooth muscles are characterized by the presence of gap junctions between the muscle cells, which allow for electrical coupling and synchronized contraction. This type of smooth muscle is often under the control of the autonomic nervous system, and its contraction is often involuntary. In the gastrointestinal tract, single unit smooth muscles are responsible for the peristaltic movements that help to propel food through the digestive system.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable (no information provided).
**Option B:** Multunit smooth muscles, on the other hand, are a type of smooth muscle tissue that contracts independently of other muscle cells, and are often found in the iris and the urinary bladder.
**Option C:** Cardiac muscles are a type of striated muscle tissue that is found in the heart and are under the control of the autonomic nervous system.
**Option D:** Skeletal muscles are a type of striated muscle tissue that are found in the voluntary muscles of the body, such as the arms and legs.

**Clinical Pearl / High-Yield Fact**
The contraction of single unit smooth muscles can be influenced by various factors such as the autonomic nervous system, hormones, and local factors such as stretch and pressure.

**Correct Answer: C. Cardiac muscles are a type of striated muscle tissue that is found in the heart and are under the control of the autonomic nervous system.