**Core Concept**
The thickness of a lead apron required to prevent radiation exposure is determined by the energy of the X-rays being used. A lead apron provides adequate protection against low-energy X-rays, but its effectiveness decreases with increasing X-ray energy.

**Why the Correct Answer is Right**
The correct answer depends on the half-value layer (HVL) of the lead apron, which is the thickness required to reduce the intensity of the X-rays by half. For low-energy X-rays (e.g., 50 kVp), a 0.5 mm lead apron is sufficient, while for high-energy X-rays (e.g., 150 kVp), a 1.5 mm lead apron is required. This is because lead is more effective at absorbing lower-energy photons.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the energy of the X-rays being used. Without this information, it is impossible to determine the required thickness of the lead apron.
**Option B:** This option is incorrect because it suggests that a 0.5 mm lead apron is sufficient for high-energy X-rays, which is not the case.
**Option C:** This option is incorrect because it suggests that a 1.5 mm lead apron is required for low-energy X-rays, which is not the case.

**Clinical Pearl / High-Yield Fact**
When using a lead apron, it is essential to ensure that it is properly fitted and that the lead is of adequate thickness to provide adequate protection against the energy of the X-rays being used.

**Correct Answer: D. 1.5 mm for high-energy X-rays**