**Core Concept**
Nonunion is a complication of fracture healing characterized by the failure of the bone to achieve bony union, resulting in a persistent fracture gap. This occurs due to an imbalance between the formation and resorption of bone tissue, leading to a lack of bridging callus and eventual failure of fracture healing.

**Why the Correct Answer is Right**
The correct answer is related to the biomechanical factors and vascular supply of the bone. Nonunion is more likely to occur in fractures with high energy, comminution, or in areas with poor blood supply, such as the scaphoid bone. The scaphoid bone has a dual blood supply, with the majority of its blood supply coming from the radial artery, which is more susceptible to injury. This makes it more prone to nonunion compared to other bones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as nonunion is not typically associated with fractures of the femur, as they have a high blood supply and tend to heal well.
**Option B:** This option is incorrect as nonunion is not typically associated with fractures of the clavicle, as they have a good blood supply and tend to heal well.
**Option C:** This option is incorrect as nonunion is not typically associated with fractures of the humerus, as they have a good blood supply and tend to heal well.

**Clinical Pearl / High-Yield Fact**
The scaphoid bone is the most common site for nonunion due to its unique anatomy and blood supply. It is essential to diagnose and treat scaphoid fractures promptly to prevent nonunion and subsequent complications.

**Correct Answer:** D.