**Core Concept**
Osteosarcoma and myositis ossificans are two distinct bone-related conditions that need to be differentiated based on radiological and clinical features. Osteosarcoma is a primary malignant bone tumor, while myositis ossificans is a benign condition characterized by heterotopic ossification within muscles.

**Why the Correct Answer is Right**
The correct answer can be determined by understanding the radiological characteristics of osteosarcoma and myositis ossificans. Osteosarcoma typically presents with a characteristic "sun-ray" or "Codman's triangle" appearance due to the production of new bone tissue by the tumor cells. In contrast, myositis ossificans typically shows a more peripheral and eccentric location, with a "heterotopic bone formation" pattern that tends to mature over time.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because osteosarcoma usually presents with a more aggressive and destructive pattern, not a "well-defined" lesion.

**Option B:** This option is incorrect because myositis ossificans typically shows a more gradual and maturing pattern of bone formation, not a "mixed lytic and sclerotic" appearance.

**Option C:** This option is incorrect because osteosarcoma often shows a more extensive and aggressive involvement of the surrounding soft tissues, not a "well-circumscribed" lesion.

**Clinical Pearl / High-Yield Fact**
The "sun-ray" appearance of osteosarcoma is a classic radiological feature that should raise suspicion for this malignancy.

**Correct Answer:** C.