  Hadronic decay modes $D^0\to(\bar K^0, \bar K^{*0})\eta,\eta'$ and
$(D^+,D_s^+)\to(\pi^+,\rho^+)\eta,\eta'$ are studied in the generalized
factorization approach. Form factors for $(D,D_s^+)\to(\eta,\eta')$ transitions
are carefully evaluated by taking into account the wave function normalization
of the eta and eta'. The predicted branching ratios are generally in agreement
with experiment except for $D^0\to\bar K^0\eta', D^+\to\pi^+\eta$ and
$D_s^+\to\rho^+\eta'$; the calculated decay rates for the first two decay modes
are too small by an order of magnitude. We show that the weak decays $D^0\to
K^-\pi^+$ and $D^+\to K^+\bar K^0$ followed by resonance-induced final-state
interactions (FSI), which are amenable technically, are able to enhance the
branching ratios of $D^0\to\bar K^0\eta'$ and $D^+\to\pi^+\eta$ dramatically
without affecting the agreement between theory and experiment for $D^0\to\bar
K^0\eta$ and $D^+\to\pi^+\eta'$. We argue that it is difficult to understand
the observed large decay rates of $D_s^+\to \rho^+\eta'$ and $\rho^+\eta$
simultaneously; FSI, W-annihilation and the production of excess eta' from
gluons are not helpful in this regard. The large discrepancy between the
factorization hypothesis and experiment for the ratio of $D_s^+\to\rho^+ \eta'$
and $D_s^+\to\eta' e^+\nu$ remains as an enigma.
