Question

light from a reflection nebula often appears blue because
it has been doppler shifted to longer wavelengths.
it is mostly hydrogen which emits blue light when excited.
it has been doppler shifted to shorter wavelengths.
interstellar dust scatters blue light more easily than red light.

Explanation:

Reflection nebulae are clouds of dust that reflect the light of nearby stars. Interstellar dust particles are small in size, and according to Rayleigh - scattering principles, they scatter shorter - wavelength (blue) light more efficiently than longer - wavelength (red) light. Doppler shift is related to motion and not the main reason for the blue color here, and hydrogen in reflection nebulae doesn't primarily emit blue light to account for the color.

Answer:

D. interstellar dust scatters blue light more easily than red light.