Final answer:
The mass distribution of a spiral galaxy is primarily determined by observing the Doppler shifts in light from stars, revealing information about rotation and dark matter.
The correct option is D.
Step-by-step explanation:
The primary way in which we determine the mass distribution of a spiral galaxy is through the measurement of the velocities of stars relative to their galaxies by observing the Doppler shift in the emitted light, usually using the hydrogen spectrum. These measurements help indicate the rotation of a spiral galaxy and provide evidence for the amount of dark matter.
A graph showing velocity versus distance from the galactic center typically reveals that the velocity stays constant instead of decreasing as expected if the mass were solely concentrated in luminous stars. This observation suggests the existence of a massive galactic halo of dark matter extending beyond the visible stars.
Other methods such as X-ray imaging of galaxy clusters reveal the presence of otherwise unseen hot clouds of ionized gas, which also correlate with dark matter distribution.
The correct option is D.