Explanation:

- The word "incredible" generally means something that is too extraordinary or improbable to be believed.

- Option 1: Unwilling - This refers to a lack of willingness or reluctance. It doesn't relate to the concept of believability.

- Option 2: Unbelievable - This is directly synonymous with "incredible," as it suggests something that is hard to believe or extraordinary.

- Option 3: Unnoticed - This means something that hasn't been observed or perceived, which is unrelated to credibility or likelihood.

- Option 4: Disturbing - This implies causing discomfort or concern, not directly linked to the concept of being hard to believe.