Which sentence uses correct verb-noun agreement?

Key rule: The phrase "The number of + plural noun" is treated as a singular noun phrase and takes the singular verb "has."

• Correct sentence: The number of books has increased significantly.

• Why the other sentences are incorrect:

  • "The number of books have increased significantly." — Incorrect because "The number of" is singular and requires the singular verb "has," not "have."

  • "A number of books has increased significantly." — Incorrect because "A number of" functions as a plural subject meaning "many books" and therefore requires a plural verb: use "have increased."

  • "A number of books is increased significantly." — Incorrect structure: "is increased" suggests a passive construction and does not convey the intended meaning. For the active meaning use "have increased," or use the passive "have been increased" only if you mean someone increased the number of books.