**Core Concept**
The question is testing the student's understanding of the units used to measure radiation, specifically the gray (Gy) and its relationship to other units such as the rad and the sievert (Sv). The gray is a unit of absorbed radiation dose, which is a measure of the energy deposited by ionizing radiation in a mass of tissue.

**Why the Correct Answer is Right**
One gray (Gy) is equal to the absorption of one joule of radiation energy per kilogram of matter. This unit is often used to express the dose of radiation absorbed by the body, such as in radiation therapy or in assessing the effects of radiation exposure. To convert from gray to other units, we can use the following conversions: 1 Gy = 100 rad and 1 Gy = 1 Sv (when considering the dose equivalent, which takes into account the biological effect of the radiation).

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, as this option is missing.
**Option B:** Incorrect, as this option is also missing.
**Option C:** Incorrect, as this option is also missing.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the gray is a unit of absorbed dose, whereas the sievert (Sv) is a unit of dose equivalent, which takes into account the biological effect of the radiation. This distinction is crucial in radiation therapy, as it helps to minimize the risk of radiation-induced harm to patients.

**Correct Answer: D. One gray (Gy) is equal to the absorption of one joule of radiation energy per kilogram of matter.**