**Core Concept**
The question is testing the knowledge of common fracture types in the elderly population, specifically in the context of a fall onto an outstretched hand. This type of injury is associated with a specific pattern of bone displacement and fracture characteristics.

**Why the Correct Answer is Right**
The correct answer is a distal radius fracture, also known as a Colles' fracture. This type of fracture occurs when the radius bone in the forearm breaks due to a fall onto an outstretched hand. The mechanism of injury involves a combination of axial load and bending force, leading to a transverse or short oblique fracture line. The distal fragment typically displaces dorsally, resulting in a characteristic "dinner fork" deformity.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because a scaphoid fracture is a common wrist injury, but it is not the most common fracture in the elderly population following a fall onto an outstretched hand.

**Option B:**
This option is incorrect because a navicular fracture is a rare and often occult injury, not typically associated with a fall onto an outstretched hand.

**Option C:**
This option is incorrect because a humeral shaft fracture can occur from a fall onto an outstretched hand, but it is less common than a distal radius fracture in the elderly population.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a distal radius fracture in the elderly population may be associated with a higher risk of osteoporosis, malunion, and complications such as median nerve injury or complex regional pain syndrome.

**Correct Answer: C. Distal radius fracture**