**Core Concept**
Myositis ossificans is a condition characterized by the abnormal formation of bone tissue within muscle, typically as a result of trauma or injury. The histological appearance of myositis ossificans is crucial in distinguishing it from other conditions, such as osteosarcoma or fibrodysplasia ossificans progressiva.

**Why the Correct Answer is Right**
The histological appearance of myositis ossificans is characterized by the presence of immature bone tissue, including osteoid and woven bone, within the muscle. This is often accompanied by a mixture of inflammatory cells, such as macrophages and lymphocytes, and the presence of cartilage or fibrous tissue. The key feature is the presence of osteoid, which is a type of unmineralized bone matrix that is often seen in conditions where there is abnormal bone formation. The histological appearance of myositis ossificans is often compared to that of osteosarcoma, but the key difference is the presence of osteoid and the absence of atypical cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Myositis ossificans does not typically mimic the histological appearance of fibrodysplasia ossificans progressiva, which is characterized by the formation of bone and cartilage in soft tissues, but also involves the presence of atypical cells and a more complex histological pattern.
**Option B:** Myositis ossificans does not typically mimic the histological appearance of osteomyelitis, which is characterized by the presence of inflammatory cells and the formation of granulation tissue, but not the formation of bone tissue within muscle.
**Option C:** Myositis ossificans does not typically mimic the histological appearance of rhabdomyosarcoma, which is a type of muscle cancer that is characterized by the presence of atypical cells and a complex histological pattern.

**Clinical Pearl / High-Yield Fact**
Myositis ossificans is often seen in young athletes who have experienced a traumatic injury, and the histological appearance can be used to distinguish it from other conditions. A key feature to remember is the presence of osteoid, which is a type of unmineralized bone matrix that is often seen in conditions where there is abnormal bone formation.

**Correct Answer: D. Osteosarcoma.**