Final answer:
When music suddenly changes tempo, it is called 'subito', which describes an immediate change in speed. Allegro, Pizzicato, and Staccato do not refer to the tempo change.
Step-by-step explanation:
When a piece of music slows down or speeds up suddenly, it is called subito. This term is used to indicate an immediate change in the tempo of the music. None of the other options provided refer to changes in speed or tempo: A) Allegro is a tempo marking that means 'fast', B) Pizzicato refers to a playing technique where strings are plucked with a finger, and C) Staccato indicates that notes should be played sharply and detached. Therefore, the correct answer is D) Subito.