Which method of separation is used for separating heavier and lighter…

Concept: A separation method is chosen based on the physical property that differs between the mixture's components. When a mixture of similarly-sized solid particles differs in mass, directing a current of air over it displaces the lighter, lower-mass component more than the heavier one -- the lighter component is carried away by the air while the heavier component, having greater mass, falls close to where it started. This air-driven separation of heavier and lighter solids is called winnowing (nishpavan/phatakna).

Application: The question describes separating heavier and lighter components purely by directing air at the mixture. That is exactly the definition of winnowing -- traditionally used after harvesting to separate heavier grain from lighter husk/chaff by tossing the mixture into the wind.

Cross-check -- how the other listed methods work instead:

• Threshing loosens grain from harvested stalks by beating or trampling them -- a mechanical, contact-based action on the stalks, not an air current directed at a mixture.

• Sieving passes a mixture through a mesh with fixed-size holes -- particles are sorted by size, with smaller ones falling through and larger ones retained, not by an air current.

• Hand picking relies on a person visually identifying and manually removing individual unwanted pieces -- a person-driven, one-at-a-time action, not an air current applied to the whole mixture.

Result: Since the question specifically describes separation by directing air at the mixture, rather than by size, manual sorting, or beating stalks, the method being asked about is winnowing.