Final answer:
The terms for when music slows down or speeds up gradually are Ritardando and Accelerando, but among the provided choices, 'Rubato' is the term that is somewhat related to changes in tempo.
Step-by-step explanation:
When a piece of music slows down or speeds up gradually, it is referred to as either a Ritardando (slowing down) or an Accelerando (speeding up). These terms are used to indicate a change in tempo, and while they are not listed in the multiple-choice options provided, the term most closely related to a change in tempo would be 'Rubato' (A). Rubato allows performers to deviate from strict tempo to express more feeling by slightly speeding up or slowing down at their discretion. However, 'Rubato' doesn't strictly mean to gradually slow down or speed up throughout a piece. Options B) Crescendo and C) Diminuendo refer to dynamics, meaning a gradual increase and decrease in volume, respectively. Option D) Legato indicates that notes are played smoothly and connectedly without separation.