The correct answer is d) they cannot
repair thymine dimers.
Thymine
dimers (or pyrimidine dimers) are formed in DNA via photochemical reactions
(exposure to UV light). Usually, those changes inside the DNA are recognized by
DNA repair enzymes and fixed by photoreactivation
(opposed mechanism of photochemical reaction). People with Xeroderma pigmentosum have a genetic
disorder of that DNA repair and thus,
are unable to repair damage caused by UV light.