D. Base Analogue
This question was addressed to me during a job interview.
This interesting question is from Repair of DNA Damage: MMR, BER in chapter DNA Mutation, DNA Repair, Transposable Elements of Cytogenetics
Easiest explanation: 6,4 Pyrimidine photoproducts are formed when the DNA is exposed to strong radiation like UV or X-rays. These can be repaired by BER.