Final answer:
When the tempo of music increases, it is called Accelerando. This is not to be confused with Ritardando, which means to slow down.
Step-by-step explanation:
The term used when the tempo speeds up during a piece of music is Accelerando (A). This contrasts with Ritardando (B), which signifies a slowing down of the tempo. Legato (C) and Staccato (D) refer to styles of playing notes smoothly connected or sharply detached, respectively, but are not related to the speed changes in the tempo of the music.