To solve this problem, we need to understand the structure of iron carbonyl, $Fe(CO)_5. \newline$Iron carbonyl is a type of metal carbonyl compound, where carbon monoxide (CO) acts as a ligand.$\newline$Let's analyze the structure of $Fe(CO)_5: \newline$• $Fe(CO)_5$ consists of one iron (Fe) atom surrounded by five carbon monoxide (CO) ligands.$\newline$• The central iron atom is bonded to each CO ligand through a coordinate covalent bond.$\newline$• The geometry of $Fe(CO)_5$ is trigonal bipyramidal, with three CO ligands in the equatorial plane and two in the axial positions.$\newline$• Since there is only one iron atom in the molecule, $Fe(CO)_5$ is classified as mononuclear.$\newline$Analyzing the options:$\newline$• Option 1: Trinuclear - This would imply three metal atoms, which is incorrect.$\newline$• Option 2: Mononuclear - This is correct as there is only one iron atom.$\newline$• Option 3: Tetranuclear - This would imply four metal atoms, which is incorrect.$\newline$• Option 4: Dinuclear - This would imply two metal atoms, which is incorrect.$\newline$Therefore, the correct option is Option 2: Mononuclear.