**Core Concept**
The quadriceps muscle group is responsible for knee extension and locking of the knee joint. This muscle group consists of four muscles: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The quadriceps muscle plays a crucial role in knee function, and damage to any of these muscles can lead to knee instability and difficulty walking.

**Why the Correct Answer is Right**
The quadriceps muscle is responsible for knee extension, which is the action of straightening the knee joint. When the quadriceps muscle contracts, it pulls the tibia (shinbone) backward, extending the knee joint. In this patient, the inability to unlock the knee joint suggests a problem with knee extension, which is a function of the quadriceps muscle. The rectus femoris muscle, one of the quadriceps muscles, is particularly involved in knee extension and is also responsible for hip flexion. Damage to the rectus femoris muscle would lead to weakness in knee extension, making it difficult to unlock the knee joint.

**Why Each Wrong Option is Incorrect**
* **Option A:** The hamstrings are responsible for knee flexion, not extension. Damage to the hamstrings would make it difficult to bend the knee, not extend it.
* **Option B:** The gastrocnemius muscle is a calf muscle that is involved in ankle movement, not knee movement. Damage to the gastrocnemius muscle would lead to weakness in ankle movement, not knee extension.
* **Option D:** The popliteus muscle is involved in knee rotation and is not primarily responsible for knee extension. Damage to the popliteus muscle would lead to weakness in knee rotation, not extension.

**Clinical Pearl / High-Yield Fact**
The quadriceps muscle is often damaged in power lifters due to repetitive strain on the muscle. This is often due to the "locking" of the knee joint during heavy lifting, which can lead to micro-tears in the quadriceps muscle. A mnemonic to remember the quadriceps muscle group is "R-V-V-V-R" which stands for Rectus Femoris, Vastus Lateralis, Vastus Medialis, Vastus Intermedius, and Rectus Femoris.

**Correct Answer:** C.