  The nonleptonic weak decays of charmed mesons into a scalar meson and a
pseudoscalar meson are studied. The scalar mesons under consideration are
$\sigma$ [or $f_0(600)$], $\kappa$, $f_0(980)$, $a_0(980)$ and $K^*_0(1430)$. A
consistent picture provided by the data implies that the light scalars below or
near 1 GeV form an SU(3) flavor nonet and are predominately the $q^2\bar q^2$
states, while the scalar mesons above 1 GeV can be described as a $q\bar q$
nonet. Hence, we designate $q^2\bar q^2$ to $\sigma, \kappa, a_0(980),
f_0(980)$ and $q\bar q$ to $K^*_0$. Sizable weak annihilation contributions
induced from final-state interactions are essential for understanding the data.
Except for the Cabibbo doubly suppressed channel $D^+\to f_0K^+$, the data of
$D\to\sigma\pi, f_0\pi, f_0K, K^*_0\pi$ can be accommodated in the generalized
factorization approach. However, the predicted rates for $D\to a_0\pi, a_0K$
are too small by one to two orders of magnitude when compared with the
preliminary measurements. Whether or not one can differentiate between the
two-quark and four-quark pictures for the $f_0(980)$ produced in the hadronic
charm decays depends on the isoscalar $f_0-\sigma$ mixing angle in the $q\bar
q$ model.
