In MnO42−, manganese is in +6 oxidation state, which is having the highest stability.
MnO4−2⇒x+4(−2)=−2
x=6
Cu+⇒x=+1 It will disproportionate in aqueous solution.
Cr+2⇒x=+2 It will also disproportionate in aqueous solution.
MnO4−3⇒x+4(−2)=−3 Mn is not stable in +5 oxidation state.
x=5
Higher the oxidation state, more is the stability of the species.