${\mathrm{KClO}}_3$ on heating decomposes into $KCl$ and $O_2$. If some ${MnO}_2$ is added the reaction goes much faster because

Without a catalyst, heating potassium chlorate causes it to change into potassium perchlorate, which, when heated further, breaks down into oxygen and potassium chloride.

The reaction can be expressed as:

$$\begin{gathered} 4{\mathrm{KClO}}_3\to 3{\mathrm{KClO}}_4+\mathrm{KCl} \\ {\mathrm{KClO}}_4\to \mathrm{KCl}+2{\mathrm{O}}_2 \end{gathered}$$

However, in the reaction below:

$2{\mathrm{KClO}}_3\left(\mathrm{s}\right)\to 2\mathrm{KCl}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)$

Hence, the correct option is (D).