**Core Concept**
The question is testing the understanding of radiation measurement units, specifically the traditional unit of radiation exposure. The correct answer is related to the amount of energy deposited by ionizing radiation in a mass of tissue.

**Why the Correct Answer is Right**
The correct answer represents the traditional unit of radiation exposure, which is used to measure the amount of energy deposited by ionizing radiation in a mass of tissue. This unit is based on the amount of energy absorbed per unit mass of tissue, typically measured in grays (Gy) or rads. However, the traditional unit is specifically related to the amount of ionization caused by radiation, which is measured in rads. One rad is equivalent to 100 ergs of energy deposited per gram of tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a recognized unit of radiation exposure.
**Option B:** This option is incorrect because it is a unit of absorbed dose, but not the traditional unit of radiation exposure.
**Option C:** This option is incorrect because it is a unit of activity, not a unit of radiation exposure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the traditional unit of radiation exposure is related to the amount of ionization caused by radiation, which is measured in rads. This unit is being replaced by the gray (Gy) in modern medical practice.

**Correct Answer: C. Röntgen (R).**