Downstream processing method involves:

The correct option is Purification.

Downstream processing refers to the stage in biosynthetic production, particularly in biotechnology and bioprocess engineering, where the desired product is recovered and purified from the complex culture medium.

Bioprocess technology is broadly divided into two main stages:
1. Upstream Processing: This phase includes all the steps prior to the harvest of the product, such as identification of the organism, strain improvement, fermentation (culturing the cells in a bioreactor), and amplification of the desired biological material.
2. Downstream Processing: Once the fermentation is complete, this stage begins to isolate the product. It involves the separation of biomass (cells) from the culture broth, cell disruption (if the product is intracellular), concentration, and purification of the target product to make it suitable for its final application (e.g., as a drug, food additive, or industrial enzyme).

Therefore, among the given choices, purification is a key downstream processing method, while identification, amplification, and fermentation belong to upstream processing.