(i)
Cr²⁺ is strongly reducing in nature. It has a d⁴ configuration. While acting as a

reducing agent, it gets oxidized to Cr³⁺(electronic configuration, d³, the latter having a half-filled t_2g level which is a more stable configuration.

In the case of Mn³⁺ (d⁴), it acts as an oxidizing agent and gets reduced to Mn²⁺ (d⁵). This has an exactly

half-filled d-orbital and is highly stable. Hence, manganese(III) is strongly oxidizing.

(ii)
Co(II) is stable in aqueous solutions, however, in the presence of strong field complexing agents, it is oxidized to Co(III). Although the 3rd ionization energy for Co is high but the higher amount of crystal field stabilization energy overcomes this ionization energy. 

(iii)
The ions in d¹ configuration tend to lose one more electron to get into stable d⁰ configuration. Also, the hydration or lattice energy is more than sufficient to remove the only electron present in the d-orbital of these ions.

Therefore, they act as reducing agents.