Which of the following words is closest in meaning to “Pledge”?

Pledge means a solemn promise or oath.

• Correct answer - an oath: it has the same meaning as Pledge.
• plain: simple; not beautiful or decorated - wrong, it does not mean the same as Pledge.
• playful: fond of fun - wrong, it does not mean the same as Pledge.
• rough: not smooth - wrong, it does not mean the same as Pledge.