**Core Concept**
Ionizing radiation refers to high-energy electromagnetic waves or particles that have enough energy to remove tightly bound electrons from atoms, resulting in the formation of ions. This type of radiation can cause damage to living tissues and is a significant concern in medical and industrial settings.

**Why the Correct Answer is Right**
Ionizing radiation includes types such as alpha particles, beta particles, X-rays, and gamma rays. These forms of radiation are known to cause ionization, leading to the potential for cellular damage and mutations. In contrast, non-ionizing radiation does not possess enough energy to cause ionization.

**Why Each Wrong Option is Incorrect**
**Option A:** Gamma rays are a form of electromagnetic radiation with high energy, which can cause ionization and are therefore ionizing radiation.
**Option B:** X-rays are also a form of ionizing electromagnetic radiation, commonly used in medical imaging.
**Option C:** Alpha particles are high-energy helium nuclei that can cause ionization when they interact with matter and are therefore ionizing radiation.

**Clinical Pearl / High-Yield Fact**
Non-ionizing radiation, such as radiofrequency waves, microwaves, and visible light, is typically used in medical treatments like hyperthermia or laser therapy, without causing significant DNA damage or ionization.

**Correct Answer: D. Non-ionizing radiation.**