**Core Concept**
The question is testing the density of sodium channels in different types of skeletal muscle fibers. The underlying principle is the variation in sodium channel density among the different muscle fiber types, which affects their excitability and contraction properties.

**Why the Correct Answer is Right**
Type II muscle fibers, also known as fast-twitch fibers, have the highest density of sodium channels, which enables them to contract rapidly. This is because they require a high rate of depolarization to generate the rapid contractions needed for explosive movements, such as sprinting. The high density of sodium channels in Type II fibers is due to the presence of a large number of voltage-gated sodium channels (Nav1.4) on the muscle fiber surface.

**Why Each Wrong Option is Incorrect**
* **Option A:** Type I muscle fibers, also known as slow-twitch fibers, have a lower density of sodium channels compared to Type II fibers. This is because they are designed for endurance activities, such as distance running, and require a slower rate of depolarization to generate sustained contractions.
* **Option B:** Cardiac muscle fibers have a unique type of sodium channel (Nav1.5) that is different from the one found in skeletal muscle. While cardiac muscle fibers do have a high density of sodium channels, they are not as high as in Type II skeletal muscle fibers.
* **Option C:** Smooth muscle fibers have a different type of sodium channel (Nav1.1) and a lower density of sodium channels compared to skeletal muscle fibers. This is because smooth muscle fibers are designed for slow, tonic contractions and do not require a high rate of depolarization.

**Clinical Pearl / High-Yield Fact**
The density of sodium channels in muscle fibers can be affected by various factors, including exercise training and muscle disease. For example, endurance training can increase the density of sodium channels in Type I muscle fibers, allowing for more efficient energy production during sustained activities.

**Correct Answer:** C. Smooth muscle fibers have a lower density of sodium channels compared to skeletal muscle fibers.