**Core Concept**
The concept being tested is the relationship between muscle length and tension. According to the sliding filament theory, muscle contraction occurs when actin and myosin filaments slide past each other. The optimal length of a muscle is the length at which this sliding filament movement is most efficient, resulting in maximum tension generation.

**Why the Correct Answer is Right**
The optimal length of a muscle is the length at which the overlap between actin and myosin filaments is greatest, allowing for the most efficient sliding filament movement. This results in maximum tension generation due to the increased number of cross-bridge interactions between actin and myosin filaments. The optimal length is typically around 1.5 to 1.7 times the muscle's resting length.

**Why Each Wrong Option is Incorrect**
**Option A:** Active length refers to the length of a muscle when it is contracting, but it does not necessarily correspond to the length at which maximum tension is generated.
**Option B:** Resting length is the length of a muscle when it is at rest, but it is not necessarily the length at which maximum tension is generated.
**Option D:** Maximal length is not a standard term in muscle physiology, and it does not accurately describe the length at which maximum tension is generated.

**Clinical Pearl / High-Yield Fact**
Remember that the optimal length of a muscle is crucial for generating maximum tension, and this concept is essential for understanding muscle physiology and its clinical applications, such as muscle rehabilitation and exercise physiology.

**✓ Correct Answer: C. Optimal length**