Complex sentence: The host galaxies of quasars can be spirals, irregulars or ellipticals: there is a correlation between the quasar's luminosity and the mass of its host galaxy, so that the most luminous quasars inhabit the most massive galaxies (ellipticals).
Split into simpler sentences: The host galaxies of quasars can be spirals, irregulars or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).