Final answer:
Boswell based The Life of Samuel Johnson on information he gathered during his twenty-year friendship with Johnson.
Step-by-step explanation:
The correct answer is option d) information he gathered during his twenty-year friendship with Johnson.
Boswell based The Life of Samuel Johnson on the information he gathered during his twenty-year friendship with Johnson. Throughout their friendship, Boswell had numerous conversations and interactions with Johnson, which he later documented in his biography. These firsthand accounts and personal experiences provided Boswell with valuable information and insights into Johnson's life and personality.