The complex $\left[\mathrm{NiCl}_{2} \mathrm{Br}_{2}\right]^{2-}$ is a tetrahedral complex due to the fact that Nickel (Ni) in this complex is in its +2 oxidation state, and thus adopts a dsp² hybridization, resulting in a tetrahedral geometry.

If $\mathrm{Ni}^{2+}$ is replaced by $\mathrm{Pt}^{2+}$, the new complex would be $\left[\mathrm{PtCl}_{2} \mathrm{Br}_{2}\right]^{2-}$. Platinum (Pt) in its +2 oxidation state typically forms square planar complexes due to its preference for dsp² hybridization.

1. Geometry :

   
   
   • $\left[\mathrm{NiBr}_2 \mathrm{Cl}_2\right]^{2-}$ is tetrahedral.
   • $\left[\mathrm{PtBr}_2 \mathrm{Cl}_2\right]^{2-}$ is square planar.
   • Thus, replacing $\mathrm{Ni}^{2+}$ with $\mathrm{Pt}^{2+}$ changes the geometry from tetrahedral to square planar.



2. Geometrical Isomerism :

   
   
   • Tetrahedral complexes, like $\left[\mathrm{NiBr}_2 \mathrm{Cl}_2\right]^{2-}$, do not exhibit geometrical isomerism.
   • Square planar complexes, like $\left[\mathrm{PtBr}_2 \mathrm{Cl}_2\right]^{2-}$, can exhibit geometrical isomerism.
   • Therefore, geometrical isomerism would be introduced by replacing $\mathrm{Ni}^{2+}$ with $\mathrm{Pt}^{2+}$.



3. Optical Isomerism :

   
   
   • Both the $\mathrm{Ni}$-based tetrahedral complex and the $\mathrm{Pt}$-based square planar complex are optically inactive.
   • Optical isomerism is not observed in either case.



4. Magnetic Properties :

   
   
   • The magnetic properties depend on the electronic configuration of the central metal ion.
   • There will be a change in magnetic properties due to the different electronic configurations of $\mathrm{Ni}^{2+}$ and $\mathrm{Pt}^{2+}$.

Based on this information, the properties expected to change when $\mathrm{Ni}^{2+}$ is replaced by $\mathrm{Pt}^{2+}$ are:

• Geometry (from tetrahedral to square planar)
• Geometrical Isomerism (introduced in the $\mathrm{Pt}$-based complex)
• Magnetic Properties (due to different electronic configurations)

Optical isomerism remains unchanged (inactive in both cases).

Therefore, the correct answer is :

Option B: A, B, and D (Geometry, Geometrical Isomerism, and Magnetic Properties).