What Observations Convince Astronomers That the Energy Source of a Quasar

What observations convince astronomers that the energy source of a quasar is physically very small?

A) rapid fluctuations in brightness since variations over 1 day mean that the source must be less than 1 light-day across
B) instant disappearance of the quasar when occulted by the Moon's edge as the Moon moves in front of a quasar, indicating a very small source size
C) sharpness of the emission lines in their optical spectra since motions within a large source would smear out the line shapes
D) extremely small size of the image of a quasar, even from Hubble Space Telescope images and radio interferometry measurements