To make certain that (something) will occur or be the case.

Correct answer: Ensure — To make certain that something will occur or be the case.

• Ensure — To make certain that something will happen or be true. Example: "Please double-check the locks to ensure the house is secure."

• Insure — To protect financially through an insurance policy. Example: "They insured the shipment against damage."

• Assure — To tell someone something confidently to remove doubt; to give assurance. Example: "I assure you I will finish the report."

• Reassure — To restore someone's confidence or calm their doubts. Example: "The doctor reassured her that the procedure was routine."

Why the others are incorrect for this prompt: Insure concerns insurance; assure and reassure concern giving or restoring confidence to a person rather than guaranteeing that an outcome will occur.