By definitions, Reduction is defined as the gain of electrons or a decrease in oxidation state.

In the reaction, elemental chlorine (Cl₂) has an initial oxidation state of 0.

In the product, sodium chloride (NaCl), the chlorine is in the form of a chloride ion (Cl⁻), with an oxidation state of -1.

The change from an oxidation state of 0 to -1 indicates a gain of electrons and thus, Cl₂ is the species that undergoes reduction.

Consequently, the correct answer is option D.