**Core Concept**
The effective dose is a measure of the total amount of radiation absorbed by the body, taking into account the sensitivity of different organs and tissues to radiation damage. It is a critical concept in radiology, used to quantify the risk of radiation-induced harm to patients undergoing medical imaging procedures.

**Why the Correct Answer is Right**
The Sievert (Sv) is the S.I. unit of effective dose, which is a measure of the biological effect of radiation. It is calculated by multiplying the absorbed dose in grays (Gy) by a weighting factor that accounts for the relative sensitivity of different organs and tissues to radiation damage. The Sievert is used to express the risk of radiation-induced harm, such as cancer and genetic mutations, to both patients and workers in the medical field.

**Why Each Wrong Option is Incorrect**
**Option A:** Becquerel (Bq) is the S.I. unit of radioactivity, which measures the rate of decay of radioactive materials, not the effective dose.
**Option C:** Gray (Gy) is the S.I. unit of absorbed dose, which measures the amount of radiation energy deposited in a given mass of tissue, but it does not take into account the sensitivity of different organs and tissues to radiation damage.
**Option D:** Roentgen (R) is a unit of exposure, which measures the ionization of air caused by X-rays or gamma rays, but it is not a direct measure of the effective dose.

**Clinical Pearl / High-Yield Fact**
When working with radiation, it's essential to remember the ALARA principle: "As Low As Reasonably Achievable." This means that medical professionals should strive to minimize radiation exposure to patients and workers whenever possible, using the lowest possible dose to achieve the desired diagnostic or therapeutic outcome.

**✓ Correct Answer: B. Sievert**