Final answer:
UV radiation causes the formation of thymine dimers, where adjacent thymines in the DNA become covalently linked, leading to potential mutations if not repaired by nucleotide excision repair.
Step-by-step explanation:
Thymine dimers are most commonly caused by C) UV radiation. These dimers form when UV light exposure causes adjacent pyrimidines, typically thymines, in a strand of DNA to dimerize. The dimerization of two thymines interrupts the normal base pairing of DNA, which, if remaining unrepaired, can lead to mutations during DNA replication and transcription.
Fortunately, these thymine dimers can often be corrected through a process known as nucleotide excision repair, where enzymes recognize the distortion caused by the dimer and correctly replace the damaged DNA strand.