**Core Concept**
Cancellous bone grafts are obtained from cancellous bone, which is characterized by its spongy and porous structure. This type of bone tissue is rich in osteoblasts, osteoclasts, and blood vessels, making it an ideal source for bone grafting procedures. Cancellous bone grafts are often used to fill bony defects, promote bone healing, and enhance bone union.

**Why the Correct Answer is Right**
The correct answer is the iliac crest. The iliac crest is a common donor site for cancellous bone grafts due to its accessibility, rich blood supply, and abundant cancellous bone tissue. The cancellous bone from the iliac crest can be easily harvested using a bone grafting instrument, and it has a high osteogenic potential, making it suitable for various orthopedic and dental applications. The iliac crest's cancellous bone is also rich in osteoblasts, which are essential for bone formation and healing.

**Why Each Wrong Option is Incorrect**
**Option A:** The femoral head is not a common donor site for cancellous bone grafts due to its limited accessibility and potential risks of complications, such as avascular necrosis.
**Option B:** The patella is not typically used as a donor site for cancellous bone grafts due to its small size and limited availability of cancellous bone tissue.
**Option C:** The rib is not a common donor site for cancellous bone grafts due to its limited availability of cancellous bone tissue and potential risks of complications, such as pneumothorax.

**Clinical Pearl / High-Yield Fact**
When harvesting cancellous bone from the iliac crest, it is essential to preserve the cortical bone and avoid damaging the surrounding soft tissues to minimize complications and ensure optimal grafting outcomes.

**Correct Answer: C. Iliac crest**