At which location would sunlight have to pass through the thickest amount of atmosphere before reaching earth's surface?

Stratosphere

The ozone layer protects life on earth by absorbing harmful ultraviolet radiation. The ozone layer is located between seventeen kilometers and thirty-five kilometers above the earth's surface in the stratosphere (2). The ozone layer is also known as the earth's shield. This layer contains high concentrations of ozone. It is interesting to note that Ozone is made up of three oxygen molecules; water consists of one hydrogen molecule and two oxygen molecules. It absorbs about ninety-seven percent of the sun's ultraviolet rays.