Which one of the following is measured in Dobson units $(DU)$?

The organisms of Earth are well protected from harmful solar radiation by a layer of gas that covers the Earth. This layer is known as:

Assertion : $UV$ radiation causes photodissociation of ozone into $O_2$ and $O$,thus causing damage to the stratospheric ozone layer.
Reason : Ozone hole is resulting in global warming and climate change

Ultraviolet radiation from sunlight causes the reaction that produces:

The presence of ozone $(O_3)$ in the atmosphere of Earth:

What was the purpose of the $Montreal$ $Protocol$?