To solve this problem, we need to determine which pair of compounds can coexist without reacting with each other.$\newline$Let's analyze each option:$\newline$Option 1: $FeCl_3$ and $SnCl_2 \newline$• $FeCl_3$ is a strong oxidizing agent, while $SnCl_2$ is a reducing agent.$\newline$• They will react with each other, so they cannot coexist.$\newline$Option 2: $HgCl_2$ and $SnCl_2 \newline$• $HgCl_2$ is not a strong oxidizing agent, and $SnCl_2$ is a reducing agent.$\newline$• However, under certain conditions, $SnCl_2$ can reduce $Hg^{2+}$ to metallic mercury.$\newline$• Therefore, they may not coexist without reacting.$\newline$Option 3: $FeCl_2$ and $SnCl_2 \newline$• Both $FeCl_2$ and $SnCl_2$ are reducing agents.$\newline$• They do not react with each other, so they can coexist.$\newline$Option 4: $FeCl_3$ and $KI \newline$• $FeCl_3$ is a strong oxidizing agent, and $KI$ is a reducing agent.$\newline$• They will react with each other, so they cannot coexist.$\newline$Therefore, the pair of compounds that can exist together is $FeCl_2$ and $SnCl_2. \newline$This corresponds to Option 3.