**Core Concept**
Non-union refers to the failure of a fracture to heal properly, resulting in a persistent gap between the bone fragments. This complication is often due to inadequate blood supply, infection, or improper immobilization.

**Why the Correct Answer is Right**
Non-union is commonly associated with fractures of the femoral neck, particularly in older adults. This is because the blood supply to the femoral neck is tenuous, and any interruption in circulation can impede healing. Additionally, the femoral neck is subject to high stresses and strains, which can further compromise the healing process.

**Why Each Wrong Option is Incorrect**
**Option A:** While osteoporosis can contribute to an increased risk of fractures, it is not a direct cause of non-union. Osteoporosis affects bone density, but other factors such as blood supply and immobilization play a more significant role in non-union.

**Option B:** Malunion refers to the improper healing of a fracture, resulting in an abnormal alignment of the bone fragments. While malunion can occur in conjunction with non-union, they are distinct complications with different underlying causes.

**Option C:** Delayed union refers to a slower-than-expected healing process, but not necessarily the complete failure of healing seen in non-union.

**Option D:** Infection can contribute to non-union by impairing the healing process, but it is not a direct cause of this complication.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a non-union, it is essential to assess the adequacy of blood supply to the affected limb, as well as the presence of any underlying infections or metabolic disorders.

**Correct Answer:** B. Malunion.