**Core Concept**
Active tension in a muscle refers to the force generated by muscle contraction, which is essential for movement and maintaining posture. This concept is crucial in understanding muscle physiology and the factors that influence muscle contraction.

**Why the Correct Answer is Right**
Active tension in a muscle is primarily dependent on the overlap between the actin and myosin filaments, which is facilitated by the sliding filament theory. The correct option should involve the concept of the overlap between these filaments, allowing for the generation of active tension. This process is mediated by the myosin heads binding to actin and undergoing a power stroke, resulting in muscle contraction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not directly relate to the generation of active tension in a muscle. While muscle length is a factor in determining the amount of tension generated, it is not the primary factor.
* **Option B:** This option is incorrect as it refers to the force generated by passive structures, such as tendons, rather than active muscle contraction.
* **Option C:** This option is incorrect as it refers to the force generated by multiple muscle units contracting simultaneously, rather than the force generated by a single muscle unit.

**Clinical Pearl / High-Yield Fact**
The sliding filament theory is essential in understanding muscle physiology, and the overlap between actin and myosin filaments is critical in determining active tension. This concept is often tested in exams and is crucial in clinical settings where muscle function is compromised due to various diseases or injuries.

**Correct Answer: C.**