The thickness of the ozone layer is measured in:

Concept: Total-column ozone is reported using the Dobson Unit (DU), the standard measure atmospheric scientists use for how much ozone lies above a given point on Earth. One DU equals the thickness a pure column of that ozone would have if compressed to standard temperature and pressure (0.01 mm of pure ozone per DU) — the unit is named after G. M. B. Dobson, who built the spectrophotometer used to measure it via ultraviolet absorption.

Application: A typical global-average ozone column is about 300 DU, i.e. roughly a 3 mm layer of pure ozone at standard conditions. Because this measurement expresses a gas column's equivalent depth rather than a linear ground distance, an astronomical distance, or a water depth, the Dobson unit is the option that correctly captures how ozone-layer thickness is expressed.

Why the other units don't fit

• Kilometre measures large linear ground distances, not an equivalent gas-column depth.

• Fathom measures water depth (e.g. the sea), unrelated to an atmospheric gas layer.

• Light year measures astronomical distance — light's travel in a year — far too large a scale for a millimetre-scale gas column.