The painting method that warhol used most often was
oil on canvas
watercolor
silk screen
objects glued to the surface of the canvas

Andy Warhol (1928 - 1987) is a key figure of the subcultural art movement Pop Art, which happened in America in the 1950s and 1960s.  He is also one of the founders and facilitators of the New York underground meeting place "Factory", where underground artists of all industries, artists, musicians, poets gathered. Popular art based on mass production, receptive to everyone, was his obsession.  He created images of celebrities, in pop art manner and was known for that. His favorite painting technique was a silk screening, which is a form of template, which uses a pattern or design on a surface consisting of a thin sheet of cardboard, metal or other material that is carved out.

The answer is silk screen.

a.) d-day invasion of normandy.

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