The correct order of $$E^0_{M^{2+}/M}$$ values with negative sign for the four successive elements Cr, Mn, Fe and Co is

The standard reduction potentials for the given metals are:

$$E^\circ_{\mathrm{Cr^{2+}/Cr}}=-0.91\ \mathrm{V}$$

$$E^\circ_{\mathrm{Mn^{2+}/Mn}}=-1.18\ \mathrm{V}$$

$$E^\circ_{\mathrm{Fe^{2+}/Fe}}=-0.44\ \mathrm{V}$$

$$E^\circ_{\mathrm{Co^{2+}/Co}}=-0.28\ \mathrm{V}$$

Across the $$3d$$ transition series, the values of $$E^\circ_{M^{2+}/M}$$ generally become less negative from left to right.

Manganese shows an exception because $$\mathrm{Mn^{2+}}$$ possesses a highly stable half-filled $$3d^5$$ configuration. Hence, reduction of $$\mathrm{Mn^{2+}}$$ to metallic manganese is less favorable, making its reduction potential highly negative.

Considering the magnitudes of the negative potentials:

$$\mathrm{Mn}:1.18$$

$$\mathrm{Cr}:0.91$$

$$\mathrm{Fe}:0.44$$

$$\mathrm{Co}:0.28$$

Therefore, the decreasing order of the values with negative sign is

$$\mathrm{Mn > Cr > Fe > Co}$$

Hence, the correct order is

$$\boxed{\mathrm{Mn > Cr > Fe > Co}}$$