**Core Concept**
The question is testing the ability to identify the characteristic mammographic appearance of a specific breast lesion. Coarse 'popcorn' calcification is a distinctive radiographic feature associated with a particular type of benign breast tumor. This calcification pattern is often seen in lesions that are composed of chondroid and fibrous tissue.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of phyllodes tumors, which are rare, usually benign breast lesions that can exhibit a wide range of histological features. The 'popcorn' calcification is thought to result from the degeneration of the chondroid matrix within these tumors, leading to the formation of coarse, irregular calcifications. This type of calcification is a key radiographic feature that helps in differentiating phyllodes tumors from other benign breast lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the characteristic mammographic appearance of phyllodes tumors. While some benign breast lesions may exhibit calcifications, the 'popcorn' pattern is specific to phyllodes tumors.

**Option B:** This option is incorrect because it is not a known mammographic feature of fibroadenomas, which are a common type of benign breast tumor. Fibroadenomas may exhibit calcifications, but they are typically smaller and more uniform than those seen in phyllodes tumors.

**Option C:** This option is incorrect because it is not a correct description of the mammographic appearance of cystosarcoma phyllodes. While this term is sometimes used to describe phyllodes tumors, it does not accurately describe the 'popcorn' calcification pattern.

**Clinical Pearl / High-Yield Fact**
Phyllodes tumors are rare breast lesions that can exhibit a wide range of histological features. They are often large and may exhibit rapid growth, but the majority are benign. The presence of coarse 'popcorn' calcification on mammogram is a key feature that suggests the diagnosis of a phyllodes tumor.

**Correct Answer:** C. Phyllodes tumors.