How does the reducing ability of the elements vary across the period from Na to Ar? It:

The reducing ability of the elements  across the period from Na to Ar decreases steadily

The capacity of a substance to reduce another substance is known as its reducing power. 

That may be accomplished either by adding or subtracting hydrogen, or by losing or gaining electrons.

The ability of a substance to diminish another substance gradually declines from sodium to argon.