**Core Concept**
In radiation protection, the use of lead aprons is crucial to prevent exposure to ionizing radiation, particularly for medical professionals and patients. The thickness of the lead apron is critical in determining its effectiveness in blocking radiation.

**Why the Correct Answer is Right**
The thickness of a lead apron is typically measured in millimeters of lead (mm Pb) and is usually around 0.5 mm Pb for general-purpose aprons. This thickness is sufficient to block 99% of X-rays with energies up to 200 keV. The lead apron works by absorbing radiation, with the lead atoms interacting with the X-ray photons to produce secondary electrons that are absorbed by the apron material. The effectiveness of the lead apron depends on the energy of the X-rays, with higher energy X-rays requiring thicker aprons for adequate blocking.

**Why Each Wrong Option is Incorrect**
**Option A:** A thickness of 0.1 mm Pb would be insufficient to block a significant amount of radiation, as it would only absorb a small percentage of X-rays.
**Option C:** A thickness of 1.5 mm Pb would be excessive for general-purpose aprons, as it would add unnecessary weight and bulk, while still providing adequate protection.
**Option D:** There is no standard thickness for a lead apron of 2 mm Pb, and this option is likely an error or a distractor.

**Clinical Pearl / High-Yield Fact**
When selecting a lead apron, it's essential to consider the energy range of the X-rays being used, as well as the duration of exposure. A general-purpose apron with a thickness of 0.5 mm Pb is usually sufficient for most radiographic procedures.

**Correct Answer: B. 0.5 mm Pb**