Which process is an oxidation?

a] 2 h+(aq) + co32-(aq) h2o(l) + co2(g)
b] fe3+(aq) fe2+(aq)
c] 2 cro42-(aq) + 2 h+(aq) cr2o72-(aq) + h2o(l)
d] ba2+(aq) + cro42-(aq) bacro4(s)
e] mno2(s) mno4-(aq

The problem has to do with oxidation states of the matter. The oxidation state of oxygen will always be -2 with the exception of peroxides which will have a state of -1. The overall balanced state of chemical compounds will be 0, so the oxidation state of Mn in MnO2 will be +4. The oxidation state of MnO4- will then be +7 to balance out to the negative one charge. The state change from +4 to +7 is 3, thus three electrons have to be lost in order for this to happen; a loss of a charge of -3 results in an increase of charge of 3. Oxidation is always the process of 'losing' electrons.

E] MnO2(s) MnO4-(aq

High-energy and unstable.