In the reaction mg(s) + 2h2o(l) mg(oh)2(s) + h2(g), what is the oxidation state of each h in h2(g)?
a. 0
b. +2 and –2
c. +1
d. –1 and +1

The rule to apply is that any element which is not combined with other element its oxidation state is 0.

So, given that in H2, each H atom is bonded only to other H atom, its oxidation state is 0.

The same is also true for O2, N2, S2, F2, Cl2, their oxidation states are 0.

Also for pure elements like Na, Li, Fe, K, Mg, Ca, in any case that the element is pure the oxidation state is 0.

option A. 0.