To solve this problem, we need to understand the reaction that occurs in Lassaigne's test when both nitrogen and sulfur are present in an organic compound.$\\$In Lassaigne's test, the organic compound is fused with sodium to form sodium cyanide (NaCN) and sodium sulfide (Na_2S).$\\$The reaction can be represented as:$\\$• $$C$+$N$+$Na$\rightarrow$NaCN$\\$• $$S$+$Na$\rightarrow$Na$_2$S$\\$When both NaCN and Na$_2$S are present, they react to form sodium thiocyanate (NaSCN):$\\$• $$NaCN$+$Na$_2$S$\rightarrow$NaSCN$+$Na$_2$C$\\$The sodium thiocyanate (NaSCN) then reacts with ferrous ions (Fe$^{2+}$) to form a blood red complex,$\\$which is known as ferric thiocyanate $[$Fe(SCN)$]^{2+}$.$\\$The reaction is as follows:$\\$• $$Fe$^{2+} +$SCN$^- \rightarrow [$Fe(SCN)$]^{2+} \\$This blood red color is characteristic of the formation of the complex $[$Fe(SCN)$]^{2+}$.$\\$Therefore, the correct answer is Option 4: $[$Fe(SCN)$]^{2+}$.