**Core Concept**
The unlocking of the knee joint during flexion involves the coordination of multiple muscles and ligaments to stabilize and move the knee. This movement is primarily facilitated by the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), which work in conjunction with the quadriceps and hamstring muscles.

**Why the Correct Answer is Right**
The unlocking of the knee is primarily facilitated by the contraction of the quadriceps muscle, which straightens the knee and moves it from an extended to a flexed position. This is achieved through the action of the quadriceps tendon, which attaches to the patella and subsequently to the tibial tuberosity. As the quadriceps contracts, it pulls the patella upward, which in turn rotates the tibia and unlocks the knee joint, allowing for flexion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically address the mechanism of knee flexion.
**Option B:** This option is incorrect because it is a distractor that does not relate to the unlocking of the knee joint during flexion.
**Option D:** This option is incorrect because it is a distractor that does not accurately describe the mechanism of knee flexion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the quadriceps and hamstring muscles work in a coordinated manner to facilitate knee movement. The quadriceps muscles are responsible for knee extension, while the hamstring muscles are responsible for knee flexion.

**Correct Answer: C. Contraction of quadriceps muscle.**