The octahedral complex $[\mathrm{Co(NH_3)_6}]^{3+}$ is diamagnetic and its central atom is sp$^3$d$^2$… actually d$^2$sp$^3$ hybridised. From this evidence, NH$_3$ is classified as a:

**Diagnose from the hybridisation.** $\mathrm{d^2sp^3}$ uses the **inner** $(n-1)\mathrm{d}$ orbitals → 'inner-orbital complex'. By VBT, this requires the ligands to *force pairing* of the metal's electrons into the lower d-set, which is only possible when the ligands induce a large crystal-field splitting — i.e. when they are **strong-field ligands**. **Diagnose from the magnetic moment.** Diamagnetic means all electrons are paired. Free $\mathrm{Co}^{3+}$ has the $\mathrm{[Ar]}\,3d^6$ configuration, which is paramagnetic when its electrons are in the high-spin arrangement. Forcing them all into the lower $\mathrm{t_{2g}}$ set (low-spin) gives the observed diamagnetism. Both lines of evidence converge: **strong-field, low-spin, inner-orbital** — option B. NH$_3$ sits high on the spectrochemical series; CN⁻ and CO sit even higher. Halides like F⁻, Cl⁻ and Br⁻ are weak-field — they make $[\mathrm{CoF_6}]^{3-}$ high-spin and paramagnetic by contrast.