First find the oxidation states of the various atoms:
in Cr2O2 2- Cr @ +1; In NH3 N @ +3; in CrO3 Cr @ +3, N2 N @ 0
Note that N gained electrons, ie, was reduced; Cr was oxidized
Now there is a problem, because B has NH4+ which the problem did not, and is not balanced, showing e- in/out
B.NH4+ → N2
Which of the following is an oxidation half-reaction?
A.Sn 2+ →Sn 4+ + 2e-
Sn lost electrons so it got oxidized
