If a chemical appears on the periodic table, it is a(n),

it’s element, it’s pure substance

Explanation:

Periodic table contains only elements and they all are pure substances..

Both are correct but for your answer element is much better

the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.

it's all four options.