Which statement defines reduction potential when considering a pair of half-cell reactions?

Reduction is the gain of electrons, then potential of reducción is the tendency of a sustance to gain electrons. In this case the sustance is an electrode.

Then the reduction potential refers to the tendency of one electrode to gain electrons and it is measured against a standard electrode which is the electrode of hydorgen.

When you consider the pair of half-cell reactions, the potential of the cell is the reduction potential - oxidation potential.