Answer:
E.) +5
Explanation:
Oxygen always has -2 oxidation number.
Because there are 3 oxygen atoms present, this means oxygen is contributing a -6 charge (-2 x 3 = -6).
Therefore, since the overall molecule is -1, chlorine must have an oxidation number of +5 to cancel all of the negative charges but 1.
You can also think of the problem like an equation. In this equation, "x" is the oxidation number of chlorine, (-2) is the oxidation number of oxygen, (3) is the number of oxygen atoms present, and the equation is set equal to (-1) because that is the overall charge of the molecule.
x - 2(3) = -1
x - 6 = -1
x = 5
