Assertion: The primitive atmosphere was reducing once i.e., without oxygen.
Reason: In the primitive atmosphere, oxygen was involved in forming ozone.

Assertion (A) is true but Reason(R) is not correct.

The lightest atoms of nitrogen, carbon, etc. formed the primitive atmosphere. Hydrogen atoms were most numerous and most reactive in primitive atmosphere. Hydrogen atoms combined with all oxygen atoms to form water leaving no free oxygen. Thus primitive atmosphere was reducing (without free oxygen) unlike the present oxidizing atmosphere (with free oxygen). Formation of ozone layer is the consequence of modern oxidizing atmosphere having plenty of free oxygen. As more oxygen accumulated in the atmosphere. Due to photosynthesis, ozone began to appear in the top layers.