Correct Answer - D
The molecular clock is the concept that during evolution the number of subsitutions in the nucleotides or nucleic acids (DNA or RNA) and hence in the protein encoded by nucleic acid, is proportional to time. The molecular clock uses fossil constraints and rates of molecular change to deduce the time in geologicv history when two species or other taxa diverged. It is used to estimate the time of occurrence of evetns called speciationor radiation.