Scientific knowledge becomes more reliable when it is
biased
questioned
not tested
not observed

questioned

Explanation:

A scientific knowledge becomes more reliable when it is tested. This is because of the fact that framing a scientific question related to the query about the natural phenomena or event can increase the curiosity of finding accurate and reliable results on the basis of implementation of suitable methodology. The evidences based upon the direct observations and data obtained after the scientific procedure can make the scientific knowledge more reliable.  

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.