Downstream processing method involves:

The correct answer is Option (4) → Purification

Option (1) Identification — Incorrect. The transformed host cells are identified, for example with a selectable marker, before they are grown on a large scale. This step belongs to the selection of recombinants, not to the processing of the finished product.

Option (2) Amplification — Incorrect. Amplification means making many copies of the gene of interest, usually by PCR. It comes early in recombinant DNA technology.

Option (3) Fermentation — Incorrect. Growing the cells in the bioreactor so that they make the product is the biosynthetic stage. Downstream processing begins only after this stage is complete.

Option (4) Purification — Correct. Separation and purification of the product make up downstream processing. The product is then formulated with preservatives and goes through quality control.

NCERT Extract:
Downstream Processing – “After completion of the biosynthetic stage, the product has to be subjected through a series of processes before it is ready for marketing as a finished product. The processes include separation and purification, which are collectively referred to as downstream processing.”