The correct option to change the given exclamatory sentence into an assertive one is:An ant is a wonderful creature.

Here's why the other options don't accurately convey the original meaning:

• What is an ant wonderful creature: This sentence maintains the interrogative form and doesn't fully remove the exclamation.
• An ant is created wonderfully: This focuses on the process of creation rather than the inherent qualities of the ant.
• Wonderful creature is an ant: This inverts the subject and object, altering the emphasis on the ant's wonder.