**Core Concept**
The dose equivalent of radiation is a measure of the biological effect of ionizing radiation, taking into account the type of radiation and its energy. It is a critical concept in radiation protection and safety.

**Why the Correct Answer is Right**
The correct unit for dose equivalent is the Sievert (Sv), which is a measure of the biological effect of radiation. It is defined as the product of the absorbed dose (in Grays, Gy) and the radiation weighting factor (wR), which depends on the type of radiation. The Sievert is used to quantify the risk of radiation-induced harm to living tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Gray (Gy) is the unit of absorbed dose, which measures the energy deposited by radiation in a mass of tissue, but it does not account for the biological effect of radiation.
**Option B:** Roentgen (R) is an older unit of exposure, which measures the ionization produced by X-rays or gamma rays in air, but it is not a measure of biological effect.
**Option C:** Becquerel (Bq) is the unit of radioactivity, which measures the rate of decay of a radioactive substance, but it is not a measure of radiation dose or biological effect.

**Clinical Pearl / High-Yield Fact**
Remember that the Sievert is the correct unit for dose equivalent, and it is essential to distinguish it from other units of radiation measurement, such as Gray, Roentgen, and Becquerel.

**Correct Answer: C. Sievert (Sv)**