In what layer of the atmosphere does the ozone maximum occur?

The stratosphere

Explanation:

Approximately 90 % of the ozone layer resides in the stratosphere and rest 10 % resides in the troposphere.

Stratosphere -

It is the second major layer of the atmosphere, present exactly above the troposphere layer.

The stratosphere has a layers with warmer and cooler region, the warmer layers are present above the cooler layers.