Concept:Use "either" after a negative verb to express "none of the two."
Explanation:The sentence already has "have not," so we need a positive word that works with "not."
"Neither" already means "not one and not the other," so using it with "not" would be a double negative.
"None" is usual for more than two items, and "all" would change the meaning.
Therefore, "either" fits best: "not bought either of the two books."
Answer:They have not bought
either of the two prescribed books.
Correct option: B. either