PCR (Polymerase Chain Reaction)

PCR, or polymerase chain reaction, amplifies a selected DNA segment through repeated cycles of denaturation, primer annealing and extension. Ideally, each newly formed DNA molecule becomes a template in the next cycle, so the number of target copies approximately doubles per cycle. After n efficient cycles, the theoretical amount is close to 2^n times the starting amount, although real reactions eventually slow because of limiting reagents and other factors. Option A is correct. Halving, disappearance and conversion into stone do not describe DNA amplification.

PCR proceeds through three principal steps. During denaturation, heat separates the two DNA strands. In the next step, called annealing, the temperature is lowered so that short primers form hydrogen bonds with their complementary sequences on the single-stranded template DNA. These primers provide the free 3′-OH end needed by DNA polymerase to begin extension. During extension, polymerase adds nucleotides to the primers. Therefore, option A, annealing, is correct. Boiling, filtration and drying are not recognized PCR stages and cannot explain primer-template binding.