Which Observations of the Radiation from Quasars Indicate That Quasars

Which observations of the radiation from quasars indicate that quasars are physically very small objects compared with galaxies?

A) their emission, which is mostly in the infrared and radio range
B) their starlike appearance on photographs, showing no structure
C) rapid fluctuations in output, often in less than 1 day
D) very high redshift of their light