The four decay modes $D^{0}\to����^{0}$, $D^{0}\to����$, $D^{+}\to����^{+}$, and $D^{+}\to��K^{+}$ are studied by using a data sample taken at the centre-of-mass energy $\sqrt{s} = 3.773$~GeV~with the BESIII detector, corresponding to an integrated luminosity of 2.93 fb$^{-1}$. The branching fractions of the first three decay modes are measured to be ${\cal B}(D^{0}\to����^{0})=(1.168\pm0.028\pm0.028)\times10^{-3}$, ${\cal B}(D^{0}\to����)=(1.81\pm0.46\pm0.06)\times10^{-4}$, and ${\cal B}(D^{+}\to����^{+})=(5.70\pm0.05\pm0.13)\times10^{-3}$, respectively, where the first uncertainties are statistical and the second are systematic. No significant signal is observed of the decay $D^+\to��{K^+}$ and the upper limit of its branching fraction is set for the first time, to be $2.1\times10^{-5}$ at the $90\%$ confidence level. The ratio of ${\cal B}(D^0\to����^0)$ to $\cal B(D^+\to����^+)$ is calculated to be $(20.49\pm0.50\pm0.45)\%$, which is consistent with the theoretical prediction based on isospin symmetry between these two decay modes.