**Core Concept:** Radiation Units: In medical physics, effective dose (ED) is a measure of the total absorbed dose of ionizing radiation that would be expected to produce a radiation-induced cancer risk equivalent to that produced by a whole-body external radiation exposure. The Sievert (Sv) is the SI unit used to measure the biological effect of ionizing radiation. Effective dose is calculated as a weighted average of the absorbed doses in different tissues, based on their radiation sensitivity. The unit for effective dose is Sievert (Sv).

**Why the Correct Answer is Right:** The correct answer, Sievert (Sv), is the standard unit used to measure the biological effect of ionizing radiation. The effective dose is calculated as a weighted average of the absorbed doses in different tissues, taking into account their radiation sensitivity. By expressing the effective dose in Sieverts, we can compare the biological effects of different types and levels of ionizing radiation accurately.

**Why Each Wrong Option is Incorrect:**

A. Sieverts (Sv) is a measure of absorbed dose, not effective dose. This option is incorrect because it does not represent the biological effect of radiation.

B. Gray (Gy) is another unit of absorbed dose, similar to Sievert (Sv). However, it is not appropriate for the question because it does not account for the differential radiation sensitivity of different tissues, which is essential for effective dose calculation.

C. Rad is the unit for absorbed dose, but it has been replaced by Sievert (Sv) as it does not account for tissue weighting factors. The question specifically asks for the unit of effective dose, which requires consideration of radiation sensitivity in different tissues.

D. Sieverts (Sv) is the correct unit for effective dose, as it takes into account the differential radiation sensitivity of tissues. This option is correct, but B is more appropriate because it explains the reasoning behind the choice of Sievert (Sv) for effective dose.

**Clinical Pearl:** The choice of Sievert (Sv) for effective dose is crucial because it reflects the biological effect of radiation on the human body, considering the differential radiation sensitivity of tissues. This is important in assessing the risk of radiation-induced cancer and other health effects. In contrast, absorbed dose (Rad or Gy) and effective dose are different concepts that reflect different aspects of ionizing radiation exposure.