**Core Concept**
The question is testing the student's knowledge of the anatomy of long bones, specifically the location of the head in relation to the epiphysis. In long bones, the epiphysis is the rounded end of the bone that forms after birth, while the head is the proximal end of the bone that articulates with another bone.

**Why the Correct Answer is Right**
The correct answer is **A. Femur** because the head of the femur is located at the epiphysis, which is the rounded end of the bone that articulates with the acetabulum of the pelvis. The femoral head is a ball-and-socket joint that allows for a wide range of motion in the hip joint. The location of the head at the epiphysis is a key anatomical feature of the femur.

**Why Each Wrong Option is Incorrect**
* **Option B:** The humerus does not have its head at the epiphysis; instead, it has a head that articulates with the glenoid cavity of the scapula.
* **Option C:** The radius and ulna do not have heads at the epiphysis; they have heads that articulate with the capitellum of the humerus and the radial notch of the ulna, respectively.
* **Option D:** The tibia and fibula do not have heads at the epiphysis; they have condyles that articulate with the femoral condyles.

**Clinical Pearl / High-Yield Fact**
In orthopedic surgery, the femoral head is a common site for hip replacement surgery, particularly in cases of osteoarthritis or hip fractures.

**Correct Answer:** A. Femur