The octahedral diamagnetic low spin complex among the following is

We need an octahedral, diamagnetic (zero unpaired electrons), low spin complex.

$$[Co(NH_3)_6]^{3+}$$: Co³⁺ has d⁶ configuration. NH₃ is a strong field ligand → low spin. In octahedral low spin d⁶: $$t_{2g}^6 e_g^0$$ → 0 unpaired electrons → diamagnetic. ✓ Octahedral ✓

$$[CoF_6]^{3-}$$: F⁻ is a weak field ligand → high spin d⁶: $$t_{2g}^4 e_g^2$$ → 4 unpaired → paramagnetic. ✗

$$[CoCl_6]^{3-}$$: Cl⁻ is a weak field ligand → high spin → paramagnetic. ✗

$$[NiCl_4]^{2-}$$: This is tetrahedral, not octahedral. ✗

The correct answer is Option 1: $$[Co(NH_3)_6]^{3+}$$.