**Core Concept**
The primary motor cortex, responsible for voluntary movements, is divided into four areas based on the body regions they control: face (area 4), arm (area 6), leg (area 4), and trunk (area 7). These areas are located in the precentral gyrus of the frontal lobe.

**Why the Correct Answer is Right**
Area 4 is responsible for controlling the face and leg, while area 6 is responsible for controlling the arm. This is due to the organization of the motor cortex in a somatotopic manner, where adjacent neurons control adjacent body parts. The face and arm are adjacent body parts, thus they are controlled by adjacent areas (4 and 6). This somatotopic organization allows for precise control over movements.

**Why Each Wrong Option is Incorrect**
**Option B:** Areas 1, 2, and 3 are located in the postcentral gyrus and are responsible for sensory processing, not motor control.
**Option C:** Areas 5 and 7 are involved in sensory processing and integration, not motor control.
**Option D:** Area 16 is involved in visual processing, and area 18 is involved in visual processing and recognition, neither of which are related to motor control.

**Clinical Pearl / High-Yield Fact**
The motor homunculus, a somatotopic map of the body on the motor cortex, helps to illustrate the organization of the primary motor cortex and its relationship to voluntary movements.

**✓ Correct Answer: A. 4 & 6**