Consider the redox reaction below.

2al(s) + 6hcl(aq) > 2alcl3(aq) + 3h2(g)

which statement correctly describes a half-reaction that is taking place?

a) hydrogen is oxidized from +1 to 0.
b) chlorine is reduced from –1 to 0.
c) aluminum is oxidized from 0 to +3.
d) hydrogen is reduced from 0 to –1.

The right answer is C.
Oxidation is the loss of electrons. A loss of electrons will appear as an increase in the positive charge of the element as it is converted to an ion. Here we have aluminum have an oxidation state equals zero as a reactant because it is in the element state. After reacting, it combines with three atoms chlorine where each chlorine atom usually has an oxidation state equals -1, therefore, we have -3 charges which have to be neutralized with the 3+ charges of aluminum.