**Core Concept**
The obturator externus muscle is one of the muscles of the hip, and its primary function is to stabilize the hip joint and assist in external rotation of the femur. This muscle plays a crucial role in maintaining the stability of the hip and facilitating movements such as abduction and external rotation.

**Why the Correct Answer is Right**
The obturator externus muscle originates from the outer border of the obturator membrane and the surrounding bone, and it inserts into the greater trochanter of the femur. The muscle's primary action is to assist in external rotation of the femur, which is essential for various hip movements, including abduction and flexion. The obturator externus muscle also helps to stabilize the hip joint, preventing excessive movement and maintaining proper alignment.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the primary action of the obturator externus muscle.
* **Option B:** This option is incorrect because the psoas major muscle is primarily involved in flexion of the hip joint, not external rotation.
* **Option C:** This option is incorrect because the gluteus minimus muscle is primarily involved in abduction and medial rotation of the hip joint, not external rotation.

**Clinical Pearl / High-Yield Fact**
The obturator externus muscle is often associated with a common clinical condition known as obturator nerve entrapment, which can cause pain and weakness in the hip and thigh. This is an important consideration for clinicians when evaluating patients with hip or thigh pain.

**Correct Answer:** C. External rotation of the femur.