Final answer:
Answer is Option D.Reflection nebulae occur when starlight is scattered by interstellar dust, typically appearing blue due to the scattering efficiency at blue wavelengths.
Step-by-step explanation:
The correct answer to how emission and reflection nebulae occur is d. Emission nebulae occur due to the ionization of gases, while reflection nebulae form by the scattering of starlight by interstellar dust. Stars with temperatures higher than about 25,000 K emit enough ultraviolet radiation to ionize nearby gases. These ionized gases then emit their own light, which results in the creation of an emission nebula, often visible as a red glow due to hydrogen gas emitting at the wavelength of the first line of the Balmer series.
On the other hand, reflection nebulae, which usually appear blue, occur when starlight is scattered by the small dust particles within a dust cloud. This scattering is more effective at blue wavelengths, which is why these nebulae often appear bluer than the stars that illuminate them.Emission nebulae are created by the ionization of gases from high-energy ultraviolet radiation, resulting in their own emitted light, often seen as red.