**Core Concept**
The question is testing the student's knowledge of the photopolymerization process in dental composites, specifically the efficiency of light sources in curing these materials. The underlying principle is the conversion of light energy into chemical bonds within the composite.

**Why the Correct Answer is Right**
The photopolymerization process in dental composites involves the activation of photoinitiators by light, which leads to the formation of free radicals. These free radicals initiate a chain reaction of polymerization, resulting in the formation of a polymer network. However, not all light is absorbed by the photoinitiator; a significant portion is lost as heat due to the inefficiencies of the light source. In the case of quartz-tungsten-halogen (QTH) lights, only a small percentage of the light is in the optimal wavelength range (typically around 450-495 nm) for curing composite resins. This limited spectral range and the heat generated by the QTH light make it less efficient for curing.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it implies a much higher percentage of usable light than is actually the case. While some QTH lights may have improved efficiency, the general consensus is that only a small percentage of the light is suitable for curing.
* **Option B:** This option is incorrect because it suggests that the percentage of usable light is higher than the actual value. The inefficiencies of QTH lights, including heat generation, reduce the amount of usable light.
* **Option D:** This option is incorrect because it implies a much lower percentage of usable light than is actually the case. While some QTH lights may have lower efficiency, the general consensus is that a small percentage of the light is suitable for curing.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the efficiency of light sources can impact the curing time and quality of the composite restoration. LED lights, for example, are generally more efficient and produce less heat than QTH lights, making them a popular choice for modern dental practices.

**Correct Answer:** B. 5-10%