Final answer:
In 'The Truth as Told by Mason Buttle', Mason's best friend who passed away at the end of fifth grade is named Benny Kilmartin.
Step-by-step explanation:
In The Truth as Told by Mason Buttle, the name of Mason's best friend who died at the end of fifth grade is Benny Kilmartin. The story centers around Mason dealing with his friend's tragic death and the subsequent social and emotional challenges he faces. The impact of Benny's death is a defining part of the storyline and Mason's journey throughout the book.