**Core Concept**
Muscle contraction involves the sliding filament theory, where actin and myosin filaments slide past each other to produce muscle shortening. During contraction, certain bands or structures within the muscle sarcomere undergo changes that can be observed under the microscope.

**Why the Correct Answer is Right**
The I-band (also known as the interband) is the region between the Z-disks of a sarcomere. During muscular contraction, the I-band decreases in length due to the sliding of the actin filaments toward the center of the sarcomere. This is because the actin filaments are attached to the Z-disks, and as the myosin filaments slide along the actin filaments, the I-band appears to decrease in length or even disappear under the microscope.

**Why Each Wrong Option is Incorrect**
* **Option A:** The A-band is the region where the myosin filaments are located and does not change length during muscle contraction.
* **Option B:** The H-zone is a region within the A-band, but it does not disappear during muscle contraction; rather, it decreases in width.
* **Option D:** The M-line is a structure located in the center of the sarcomere, but it does not disappear during muscle contraction.

**Clinical Pearl / High-Yield Fact**
The sliding filament theory is a fundamental concept in muscle physiology and is essential for understanding muscle contraction and relaxation. It's crucial to remember that muscle contraction involves the coordinated sliding of actin and myosin filaments, which can be observed under the microscope.

**Correct Answer: C. I-band**