The compounds which produce $\mathrm{CO}$ gas finally on heating are:

The reactions are,

(a) ${\mathrm{FeC}}_2{\mathrm{O}}_4\stackrel{\mathrm{\Delta }}{⟶}\mathrm{FeO}+\mathrm{CO}\uparrow +{\mathrm{CO}}_2\uparrow$

(b) ${\mathrm{SnC}}_2{\mathrm{O}}_4\stackrel{\mathrm{\Delta }}{⟶}\mathrm{SnO}+\mathrm{CO}\uparrow +{\mathrm{CO}}_2\uparrow$

(C) $\begin{array}{r}{\mathrm{Ag}}_2{\mathrm{C}}_2{\mathrm{O}}_4\stackrel{\mathrm{\Delta }}{⟶}{\mathrm{Ag}}_2\mathrm{O}+\mathrm{CO}\uparrow +{\mathrm{CO}}_2\uparrow \end{array}$

                               $∆\downarrow {300}^{\mathrm{o} }\mathrm{C}$

                                 $2\mathrm{Ag}\downarrow +\frac{1}{2}{\mathrm{O}}_2\uparrow$

${\mathrm{Ag}}_2{\mathrm{C}}_2{\mathrm{O}}_4\stackrel{{300}^{\circ }\mathrm{C}}{⟶}2\mathrm{Ag}\downarrow +2{\mathrm{CO}}_2\uparrow$

(d) ${\mathrm{HgC}}_2{\mathrm{O}}_4\stackrel{\mathrm{\Delta }}{⟶}\mathrm{HgO}+\mathrm{CO}\uparrow +{\mathrm{CO}}_2\uparrow$

                              $\mathrm{\Delta }\downarrow {400}^{\circ }\mathrm{C}$

                   $\mathrm{Hg}\uparrow +\frac{1}{2}{\mathrm{O}}_2\uparrow$   

${\mathrm{HgC}}_2{\mathrm{O}}_4\stackrel{{400}^{\circ }\mathrm{C}}{⟶}\mathrm{Hg}\uparrow +2{\mathrm{CO}}_2^{\uparrow }$

So the compounds which produce  $\mathrm{CO}$ gas finally on heating are ${\mathrm{FeC}}_2{\mathrm{O}}_4$  and ${\mathrm{SnC}}_2{\mathrm{O}}_4$.