When determining the oxidation state of an element, what indicates oxidation has occurred?

The correct choice indicates that an increase in the oxidation number signifies that oxidation has occurred. Oxidation is defined as the loss of electrons by an atom, ion, or molecule. When an element's oxidation state increases, it reflects that the element has lost electrons during a chemical reaction.

This process can be illustrated in redox reactions, where one species undergoes oxidation while another undergoes reduction. A common example is the oxidation of iron in rusting, where iron loses electrons and increases its oxidation state from 0 in elemental iron to +3 in iron(III) oxide.

Understanding oxidation in this way is fundamental in the study of chemical reactions and balancing redox equations, as you must track changes in oxidation states to understand electron flow in a reaction mechanism.