**Core Concept**
The question tests the association between radiation exposure and its effects on various organs, particularly in infants. This concept is rooted in the field of radiation oncology and the study of radiation-induced malignancies.

**Why the Correct Answer is Right**
Radiation exposure during infancy has been strongly linked to an increased risk of developing thyroid cancer. This is because the thyroid gland is highly sensitive to ionizing radiation, and exposure to high doses, especially during critical periods of development, can lead to malignant transformations. The thyroid gland's sensitivity to radiation is thought to be due to its high concentration of iodine, which is essential for thyroid hormone production and can be affected by radiation damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Breast cancer is a common malignancy, but it is not primarily associated with radiation exposure during infancy. While ionizing radiation can increase the risk of breast cancer, this effect is more pronounced with exposure at older ages, particularly during puberty or adulthood.
**Option B:** Melanoma is a type of skin cancer that is primarily associated with UV radiation exposure, not ionizing radiation. While radiation can increase the risk of various cancers, melanoma is not directly linked to radiation exposure during infancy.
**Option D:** Lung cancer is a major public health concern, but it is not primarily associated with radiation exposure during infancy. While radiation can increase the risk of lung cancer, especially with long-term exposure to radon or other carcinogens, this effect is more pronounced with occupational or environmental exposures rather than radiation exposure during infancy.

**Clinical Pearl / High-Yield Fact**
The risk of radiation-induced thyroid cancer is highest when exposure occurs before the age of 5, and the risk is particularly increased when exposure occurs during the first year of life. This highlights the importance of minimizing radiation exposure in infants and young children, particularly in medical imaging procedures.

**✓ Correct Answer: C. Thyroid**