Which of the following is a cause of mutations?

Mutations are changes in the genetic material of an organism, and they can occur due to various factors. X-rays are a form of ionizing radiation that can cause ionization of atoms within biological tissues, leading to breaks in DNA strands or alterations in nucleotides. This type of high-energy radiation has enough energy to disrupt molecular bonds and induce changes that can result in mutations.

The other choices, while they may interact with biological systems in various ways, are less directly associated with causing mutations. Thermal energy might increase the rate of molecular movements and reactions but does not directly induce DNA mutations in the same manner as X-rays. Stress, while it can have significant effects on an organism's health and well-being, does not cause genetic mutations in the same direct way as ionizing radiation. Sound waves, being a form of mechanical energy, do not have the necessary energy to cause changes in the molecular structure of DNA and, therefore, do not directly induce mutations.

Understanding the mechanisms by which different forms of energy can impact genetic material is important for studying genetics and the processes that lead to variations in populations.