As one moves away from a source of scatter radiation, the radiation dose decreases because:

The correct answer is based on the principle of radiation physics and how scatter radiation behaves as distance from the source increases. Specifically, as one moves away from a source of scatter radiation, the scatter diverges, and the intensity of radiation decreases due to the inverse square law. This law states that the intensity of radiation (or any point source radiation) diminishes with the square of the distance from the source.

The idea here is that as the distance increases, the same amount of scattered radiation is dispersed over a larger area, leading to a reduced dose at any given point farther from the source.

While it may seem logical to consider the energy of scattered photons, the overall effect of distance on radiation dose is primarily related to how radiation intensity dissipates with distance rather than energy changes. Therefore, although the energy of scattered photons may affect how they interact with matter, it is not the primary reason for the decrease in dose when moving away from the scatter source.