Answer:
This is an example of Onomatopoeia
Step-by-step explanation:
Onomatopoeia es the process in which the word and the pronunciation of it, phonetically imitates the sound of what it describes. Thwack is a short and loud sound of something hitting a surface, then this word imitates the sound of the action it is describing when it refers to the sound of the ball hitting the mitt back and forth.