The Distinct Blue Color of the Nebulosity Around Stars in Young

The distinct blue color of the nebulosity around stars in young clusters such as the Pleiades (see Figure 13-4 in the text) is caused by

A) atoms of gas emitting light by fluorescence, having been excited by ultraviolet radiation from hot stars.
B) light emitted by interstellar gases but Doppler-shifted by motion toward the observer.
C) starlight reflected by blue-colored interstellar material.
D) starlight scattered and reflected by small dust grains in the interstellar material.