Final answer:
The word 'vacuous' best completes the sentence as it means lacking in ideas or intelligence, making it appropriate for describing a person who usually has nothing meaningful to say.
Step-by-step explanation:
The sentence "A(n) vacuous person usually has nothing meaningful to say." best fits the blank. The word 'vacuous' means lacking in ideas or intelligence, which would make sense in the context of the sentence. Let's look at the other options to see why they don't fit as well: 'Abstract' refers to something that is not concrete or based on general ideas rather than specific examples. 'Candid' means straightforward and truthful. 'Diligent' refers to someone who is hardworking and pays careful attention to their work. Therefore, 'vacuous' is the correct choice.