Explanation:

Correct Option is 3:Guarentee

- Option 1: Greave

- "Greave" refers to armor for the shin, correctly spelled.

- Option 2: Groan

- "Groan" means a low, mournful sound expressive of pain or disapproval, correctly spelled.

- Option 3: Guarentee

- This is incorrectly spelled. The correct spelling is "Guarantee."

- "Guarantee" refers to a formal assurance or promise.

- Option 4: Gravely

- "Gravely" is an adverb meaning seriously or solemnly, correctly spelled.