The correct option is D [CoF6]3−
In [Fe(CN)6]4− complex, Fe2+ electronic configuration is [Ar]3d6. Since CN is strong field ligand d electrons are paired. The complex is diamagnetic.
In Ni(CO)4, the oxidation state of Ni is zero. Its electronic configuration is {Ar}3d84s2. In presence of CO, the electronic configuration of Ni is [Ar]3d104s0, all electrons are paired. The complex is diamagnetic.
In [Ni(CN)4]2− complex, the electronic configuration of Ni2+[Ar]3d84s0 due to strong field CN− ligand all electrons are paired. The complex is diamagnetic.
In [CoF6]3− complex, the electronic configuration of Co3+ is [Ar]3d6 since F is weak ligand, four electrons are unpaired. Hence the complex is paramagnetic.
Note: When all electrons are paired, the complex is diamagnetic. But if one or more electrons are unpaired, the complex is paramagnetic.