Which musical term means to decrease speed gradually?

The term that means to decrease speed gradually is "rallentando." This term instructs musicians to gradually slow down the pace of the music, allowing for a more expressive or dramatic effect. It is commonly used to indicate a deceleration in tempo, often towards the end of a piece or a specific passage, creating a feeling of relaxation or a poignant conclusion.

In contrast, other terms in the options refer to different concepts in music. "Allargando" generally means to broaden the tempo, often implying a slight slowing down but more in the context of expanding the expression of the music rather than simply decreasing speed. "Piu mosso" indicates a quicker tempo, suggesting to play faster instead of slowing down. "Meno," which means "less," generally modifies other terms, such as "meno mosso," to mean "less motion," but does not specifically indicate a gradual decrease in speed alone.

Thus, "rallentando" is the precise term for the gradual reduction in tempo, making it the correct choice for this question.