**Core Concept**
The locking mechanism of the knee joint is a complex process that involves the coordination of several muscles and ligaments. The primary function of the locking mechanism is to stabilize the knee joint and facilitate activities such as squatting and climbing stairs.

**Why the Correct Answer is Right**
The quadriceps femoris muscle, specifically the vastus medialis obliquus (VMO) and vastus lateralis (VL) components, play a crucial role in knee locking. When the knee is in a flexed position, the VMO contracts, which helps to pull the patella (kneecap) medially (inward) and facilitate the tibiofemoral joint space. This contraction, along with the contraction of the VL, helps to lock the knee joint in place, allowing for greater stability and control. The VMO and VL work in conjunction with the hamstring muscles to maintain knee stability and facilitate activities such as squatting.

**Why Each Wrong Option is Incorrect**
* **Option A:** The biceps femoris muscle, part of the hamstring group, is not directly involved in knee locking. While it does contribute to knee stability, its primary function is to extend the hip and flex the knee.
* **Option B:** The gastrocnemius muscle, a plantarflexor of the foot and flexor of the knee, is not a primary contributor to knee locking. Its primary function is to facilitate activities such as walking and running.
* **Option D:** The popliteus muscle, which helps to unlock the knee joint, is not a locking muscle. Its primary function is to rotate the tibia medially, which helps to unlock the knee joint and facilitate activities such as running and jumping.

**Clinical Pearl / High-Yield Fact**
The VMO plays a crucial role in knee locking and stability, and its dysfunction can lead to patellofemoral pain syndrome and other knee-related injuries. Therefore, it is essential to assess and strengthen the VMO as part of a comprehensive knee rehabilitation program.

**Correct Answer:** C. Quadriceps femoris muscle.