Explanation:
The stability of the +2 oxidation state in transition metals is determined by several factors, including the electronic configuration, the shielding effect, and the ionization energy.

Electronic Configuration:
The electronic configuration of the transition metals in question is as follows:
- Cr: [Ar] 3d5 4s1
- Mn: [Ar] 3d5 4s2
- Fe: [Ar] 3d6 4s2
- Co: [Ar] 3d7 4s2

Shielding Effect:
The shielding effect refers to the ability of inner electrons to shield outer electrons from the full nuclear charge. As we move across the period, the number of inner electrons remains constant, while the nuclear charge increases. This leads to a decrease in the shielding effect and an increase in the effective nuclear charge experienced by the outer electrons.

Ionization Energy:
Ionization energy is the energy required to remove an electron from an atom or ion. As we move across the period, the ionization energy generally increases due to the increased effective nuclear charge.

Explanation of the Order:
Based on the above factors, the stability of the +2 oxidation state can be explained as follows:

- Mn: Mn has an electronic configuration of [Ar] 3d5 4s2. The 4s electrons are relatively easy to remove due to their higher energy level compared to the 3d electrons. Therefore, the +2 oxidation state is relatively stable for Mn.
- Cr: Cr has an electronic configuration of [Ar] 3d5 4s1. The removal of one electron from the 4s orbital to form the +2 oxidation state leaves a half-filled 3d orbital, which is relatively stable due to the exchange energy associated with the electron-electron repulsion. Therefore, the +2 oxidation state is stable for Cr.
- Fe: Fe has an electronic configuration of [Ar] 3d6 4s2. The removal of two electrons from the 4s orbital to form the +2 oxidation state leaves a half-filled 3d orbital, which is again relatively stable. Therefore, the +2 oxidation state is stable for Fe.
- Co: Co has an electronic configuration of [Ar] 3d7 4s2. The removal of two electrons from the 4s orbital leaves a completely filled 3d orbital, which is relatively stable. Therefore, the +2 oxidation state is stable for Co.

Therefore, the correct order is Mn > Cr > Fe > Co, which corresponds to option A.