The line: "I heard the familiar "jug--jug--SPAT!" of a motorcycle, and a frantic policeman rode alongside" best demonstrates which literary device?

Onomatopoeia.

Explanation:

In the given line, the figurative language which is used is onomatopoeia.

Onomatopoeia is a literary device which is used to create phonetical sounds of any object using words. It mimics the sound by describing them in words. For instance, 'tick-tock' is an onomatopoeia for clock.

In the given line, 'jug-jug-SPAT' is an onomatopoeia used for describing the police motorcycle which followed Gatsby, in the novel 'The Great Gatsby.'

Therefore, onomatopoeia is the correct answer.