To solve this problem, we need to determine the geometry and magnetic behavior of the complex $[Ni(CO)_4]. \newline$Let's analyze the complex:$\newline$• The central metal atom is nickel (Ni), and it is in the zero oxidation state in $[Ni(CO)_4]. \newline$• Carbon monoxide (CO) is a neutral ligand and a strong field ligand.$\newline$• The electronic configuration of Ni in the zero oxidation state is $[Ar] \, 3d^{10} \, 4s^0. \newline$• In the presence of strong field ligands like CO, the 3d orbitals are fully filled, and the 4s and 4p orbitals are used for hybridization.$\newline$• The hybridization of Ni in $[Ni(CO)_4]$ is $sp^3,$ which leads to a tetrahedral geometry.$\newline$• Since all the 3d orbitals are fully filled, there are no unpaired electrons.$\newline$• Therefore, the complex is diamagnetic.$\newline$Analyzing the options:$\newline$• Option 1: Square planar geometry and paramagnetic - Incorrect.$\newline$• Option 2: Tetrahedral geometry and diamagnetic - Correct.$\newline$• Option 3: Square planar geometry and diamagnetic - Incorrect.$\newline$• Option 4: Tetrahedral geometry and paramagnetic - Incorrect.$\newline$Therefore, the correct option is Option 2.