**Core Concept**
Radiation-induced tumors are a well-documented consequence of ionizing radiation exposure. These tumors occur due to genetic mutations caused by radiation damage to the DNA of cells, leading to uncontrolled cell growth and tumor formation.

**Why the Correct Answer is Right**
The most common tumor due to radiation is the **osteosarcoma**, a type of bone cancer. Radiation-induced osteosarcoma is thought to occur due to damage to the osteogenic cells (osteoblasts) in the bone marrow. The genetic mutations caused by radiation lead to the activation of the Wnt/β-catenin signaling pathway, which promotes the proliferation and differentiation of osteoblasts into osteosarcoma cells. This type of cancer typically arises in individuals who have been exposed to high doses of radiation, such as in the treatment of childhood cancers or in individuals who have been exposed to radiation in a nuclear accident.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **chordoma** is a rare type of tumor that arises from the notochord, a precursor to the spine and skull, but it is not the most common radiation-induced tumor.

**Option B:** This option is incorrect because **glioblastoma** is a type of brain cancer that can be caused by radiation, but it is not the most common radiation-induced tumor.

**Option C:** This option is incorrect because **thyroid cancer** can be caused by radiation exposure, particularly in individuals who were exposed to radiation in childhood, but it is not the most common radiation-induced tumor.

**Clinical Pearl / High-Yield Fact**
Radiation-induced tumors can occur years or even decades after radiation exposure, and the risk of developing these tumors increases with the dose and duration of radiation exposure. It is essential for healthcare providers to be aware of the risk of radiation-induced tumors in patients who have received radiation therapy.

**Correct Answer:** D. Osteosarcoma.