Pyrimidine dimers are seen in:: (A) Xeroderma pigmentosum, (B) Alkylating agents, (C) X-rays, (D) Gamma rays
Answer is A. DNA damaging agent that results in pyrimidine dimer is U-V light chemicals:

DNA repair mechanism that repair pyrimidine dimer is Nucleotide Excision repair.
Defective Nucleotide excision repair mechanism causes Xeroderma pigmentosum, Cockayne Syndrome, Trichothiodystrophy.
So, pyrimidine dimers are seen in disorders caused by defective nucleotide excision repair.

Reference- Harper 30th edition Pg - 390