Which of these half-reactions represents reduction? I. Fe2+ → Fe3+
II. Cr2O72- → Cr3+
III. MnO4- → Mn2+

The half-reactions represents reduction are as follows.

Explanation:

A half-reaction where addition of electrons take place or a reaction where decrease in oxidation state of an element  takes place is called reduction-half reaction.

For example, the oxidation state of Cr in is +6 which is getting converted into +3, that is, decrease in oxidation state is taking place as follows.

Similarly, oxidation state of Mn in is +7 which is getting converted into +2, that is, decrease in oxidation state is taking place as follows.

Thus, we can conclude that half-reactions represents reduction are as follows.

it means there are 3 atoms of oxygen