Final answer:
The formation of pyrimidine dimers results from UV radiation (option A). Pyrimidine dimers are primarily formed as a result of exposure to UV radiation, which can link adjacent pyrimidines in DNA leading to mutations.
Step-by-step explanation:
The formation of pyrimidine dimers, such as thymine dimers, is predominantly caused by UV radiation. When DNA is exposed to UV light, adjacent pyrimidines on a strand can become covalently linked, forming dimers.
This process can lead to DNA mutations if the dimers are not repaired through nucleotide excision. Notably, this type of DNA damage does not typically result from exposure to X-rays, gamma rays, or infrared radiation.
Hence, option A is correct answer.