The ternary acid formed with chlorine that has the fewest oxygen atoms is hypochlorous acid.

Here's why:

* Ternary acids are acids containing hydrogen, oxygen, and one other element.

* Hypochlorous acid has the formula HClO. It has only one oxygen atom.

* Other chlorine-containing ternary acids include chlorous acid (HClO₂), chloric acid (HClO₃), and perchloric acid (HClO₄), each with more oxygen atoms.