**Core Concept**
The cardiac muscle has a unique property known as the **stretch-activated response**, where it tries to return to its resting length after being stretched. This is mediated by a protein that acts as a molecular sensor, detecting changes in muscle length and triggering a response to prevent overstretching.

**Why the Correct Answer is Right**
The protein responsible for preventing overstretching of cardiac muscle is **Titin**. Titin is a giant protein that spans from the Z-disk to the M-line in the cardiac muscle sarcomere. It plays a crucial role in maintaining the structural integrity of the muscle and regulating its mechanical properties. When the muscle is stretched, Titin undergoes a conformational change, which triggers a signaling pathway that helps to prevent overstretching and maintain muscle function.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because it does not specify a protein, and even if it did, it would be a generic term that is not directly related to cardiac muscle mechanics.
* **Option B:** This is incorrect because it is a type of muscle fiber, but not a protein that regulates muscle length.
* **Option C:** This is incorrect because it is a type of cardiac glycoside, which is a medication used to treat heart failure, but it is not a protein that regulates muscle length.

**Clinical Pearl / High-Yield Fact**
Remember that Titin is a key component of the cardiac muscle's **sarcomere**, and its dysfunction is associated with **dilated cardiomyopathy**, a condition characterized by weakened heart muscle and impaired cardiac function.

**Correct Answer: D. Titin**