Which of the following is the SI unit for the rad?

The SI unit for the rad is the Gray. The rad, which stands for radiation absorbed dose, is a measure of the amount of energy deposited by ionizing radiation in a material, typically in human tissue. The Gray (Gy) is the international standard unit used to quantify the same concept, and 1 Gray is equivalent to 100 rads.

In the context of radiation measurement, the Gray is favored because it is based on the International System of Units (SI), which is used globally for scientific and technical measurements. This makes it important for consistency and standardization in fields like radiology and radiation safety.

Sieverts relate to biological effect—taking into account the biological impact of radiation on living tissues—while rem is the traditional unit that encompasses the same concept but is used primarily in the United States. Roentgen measures exposure to X-rays and gamma rays specifically in air, and it is not directly equivalent to the measurement of absorbed dose that the Gray and rad provide. Thus, the Gray is the correct answer, as it is the SI unit that directly correlates with the dose measurement indicated by the rad.