The step 'B' in the figure is called as:

The correct answer is Option (3) → Annealing

Each PCR cycle has three steps: denaturation, primer annealing and extension. In step B the short primers bind to their matching sequences on the separated single strands; this binding is annealing.

Option (1) Termination — Incorrect. Termination is a stage of transcription and translation; it is not a step of PCR.

Option (2) Extension — Incorrect. In extension, Taq polymerase adds nucleotides to the primers. That is step C.

Option (3) Annealing — Correct. A PCR cycle has three steps: denaturation of the double-stranded DNA by heat, annealing of the primers to the single strands, and extension by a thermostable DNA polymerase. Step B, where the primers bind, is annealing.

Option (4) Reannealing — Incorrect. PCR has no step of this name; joining of the two original strands again would block the primers.