Final answer:
The adjective that correctly completes the sentence is 'worst'.
Step-by-step explanation:
The adjective that correctly completes the sentence is worst.
The sentence states that Brett received the worst grade in his class on the history test, implying that his grade was the lowest or poorest among his classmates.
For example, if all other students in Brett's class received higher or better grades than him, then his grade would be considered the worst.
Learn more about Adjectives to complete a sentence