The correct answer is option (A): sp3,two
Monodentate ligand is uninegative and hence, the oxidation number of Ni in [NiX4]2− is +2.
The electronic configuration of Ni2+ is [Ar]3d8.
If the compound is diamagnetic it means all the electrons are paired. Hence, there will be one 3d orbital empty and one 4s orbital empty.
The elements, in the columns of the periodic table in which d subshells are being occupied are known as d block elements.
These are the elements that have the capability of forming stable cations with incompletely filled d orbitals. Elements like mercury and Zinc are not considered transition metals because they have electronic configurations: (n-1)d10 ns2. These elements have filled d-orbitals in their ground state and, therefore, even in some of their oxidation states.