In the given complexes, $H_2O$ acts as a weak field ligand for $3d$ series elements in +2 and +3 oxidation states (with the exception of $Co^{3+}$ where it can act as a strong field ligand). Let's calculate the Crystal Field Stabilisation Energy (CFSE) for each:

1. $[Mn(H_2O)_6]^{3+}$ : Central metal is $Mn^{3+}$ ($3d^4$). As $H_2O$ is a weak field ligand, it forms a high-spin complex ($t_{2g}^3 e_g^1$). $CFSE = (-0.4 \times 3 + 0.6 \times 1)\Delta_o = -0.6\Delta_o$

2. $[Fe(H_2O)_6]^{3+}$ : Central metal is $Fe^{3+}$ ($3d^5$). As $H_2O$ is a weak field ligand, it forms a high-spin complex ($t_{2g}^3 e_g^2$). $CFSE = (-0.4 \times 3 + 0.6 \times 2)\Delta_o = (-1.2 + 1.2)\Delta_o = 0\Delta_o$

3. $[Co(H_2O)_6]^{2+}$ : Central metal is $Co^{2+}$ ($3d^7$). It forms a high-spin complex ($t_{2g}^5 e_g^2$). $CFSE = (-0.4 \times 5 + 0.6 \times 2)\Delta_o = -0.8\Delta_o$

4. $[Co(H_2O)_6]^{3+}$ : Central metal is $Co^{3+}$ ($3d^6$). With $H_2O$ it usually forms a low-spin complex ($t_{2g}^6 e_g^0$) having $CFSE = -2.4\Delta_o$. Even if considered high spin ($t_{2g}^4 e_g^2$), $CFSE = -0.4\Delta_o$.

Therefore, $[Fe(H_2O)_6]^{3+}$ exhibits zero CFSE.