When did oxygen levels start to decline from 35 percent down to today's levels?
a. after the carboniferous period
b. during the permian period
c. after the triassic period
d. before the silurian period

Oxygen levels start to decline from 35 percent down to today's level after the carboniferous period.

Option a

Explanation:

The Pangea formation marked the carboniferous period. Pangea has revolutionized land and sea, laying the foundations for new species of plants and animals. A new group of species, diapsids, enters the stage.  

During the carboniferous period, 35 percent of oxygen levels began to fall to current levels. Oxygen levels dropped from 35 percent to their current level about 95 million years ago. Oxygen is a product produced as waste from photosynthesis and used by respiration.