The step 'D' in the figure depicts:

The correct answer is Option (4) → Only amplified product

Step A is denaturation by heat, step B annealing of primers, and step C extension of the primers. Step D shows the two double-stranded copies of the target region that the cycle has produced.

Option (1) Region where the restriction enzyme cuts. — Incorrect. No restriction enzyme is used in PCR; the copied region is fixed by the primers.

Option (2) An important gene of interest — Incorrect. The gene of interest is in the starting DNA. Step D shows its copies after amplification.

Option (3) Primer joining region — Incorrect. Primers join the template in step B.

Option (4) Only amplified product — Correct. After many cycles nearly all the DNA is this amplified piece, about a billion copies after 30 cycles.