Answer
a) 0
b) +2
c) +3
d) 0
Work Step by Step
a) $V$ is a element and in standard state, all elements have an oxidation number of zero.
b) $Mg^{2+}$ is an ion and the oxidation number for ions corresponds to their charge which is +2 in this case.
c) $Cr^{3+}$ is an ion and the oxidation number for ions corresponds to their charge which is +3 in this case.
d) $O_{2}$ is a element and in standard state, all elements have an oxidation number of zero.
