Ultraviolet (UV) rays are employed in water purification due to their germicidal attributes. UV radiation inactivates or eliminates detrimental microbes by compromising their DNA, thereby establishing it as an efficient water purification technique.
Infrared rays are predominantly utilized for thermal applications.
X-rays and Gamma rays find application in medical diagnostics and oncology interventions, but are not typically applied to water purification.
Therefore, the appropriate response is Ultraviolet rays.\bigskip