  We present a set of nonlocal thermodynamic equilibrium steady-state
calculations of radiative transfer for one-year old type II supernovae (SNe)
starting from state-of-the-art explosion models computed with detailed
nucleosynthesis. This grid covers single-star progenitors with initial masses
between 9 and 29$M_{\odot}$, all evolved with KEPLER at solar metallicity and
ignoring rotation. The [OI]$\lambda\lambda$$6300,6364$ line flux generally
grows with progenitor mass, and H$\alpha$ exhibits an equally strong and
opposite trend. The [CaII]$\lambda\lambda$$7291,\,7323$ strength increases at
low $^{56}$Ni mass, low explosion energy, or with clumping. This CaII doublet,
which forms primarily in the explosively-produced Si/S zones, depends little on
the progenitor mass, but may strengthen if Ca$^+$ dominates in the H-rich
emitting zones or if Ca is abundant in the O-rich zones. Indeed, Si-O shell
merging prior to core collapse may boost the CaII doublet at the expense of the
OI doublet, and may thus mimic the metal line strengths of a lower mass
progenitor. We find that the $^{56}$Ni bubble effect has a weak impact,
probably because it is too weak to induce much of an ionization shift in the
various emitting zones. Our simulations compare favorably to observed SNe II,
including SN2008bk (e.g., 9$M_{\odot}$ model), SN2012aw (12$M_{\odot}$ model),
SN1987A (15$M_{\odot}$ model), or SN2015bs (25$M_{\odot}$ model with no Si-O
shell merging). SNe II with narrow lines and a low $^{56}$Ni mass are well
matched by the weak explosion of 9$-$11$M_{\odot}$ progenitors. The
nebular-phase spectra of standard SNe II can be explained with progenitors in
the mass range 12$-$15$M_{\odot}$, with one notable exception for SN2015bs. In
the intermediate mass range, these mass estimates may increase by a few
$M_{\odot}$ with allowance for clumping of the O-rich material or CO molecular
cooling.
