The correct answer is option 4: UV-rays. Thymidine dimer formation in DNA is a type of DNA damage that occurs specifically as a result of exposure to UV radiation. UV radiation, which is present in sunlight, has enough energy to disrupt the structure of DNA molecules. When UV radiation interacts with DNA, it can cause adjacent thymines (a base in DNA) to bind together, forming what is called a thymidine dimer. This undesired bond formation between adjacent thymines can lead to distortions in the DNA helix and interfere with DNA replication and transcription processes. It is important to note that options 1, 2, and 3 (IR-rays, X-rays, and bandg-rays) are not known to directly cause thymidine dimer formation in DNA. Alert - correct answer should be option 4: UV-rays.