Final answer:
The defense mechanism being used by the young man who states "No, I don't believe it. It can't be true." in response to his mother's accident is denial.
Step-by-step explanation:
The young man who responds to the news of his mother's serious automobile accident is using denial as a defense mechanism. Denial is a type of defense mechanism where an individual refuses to accept reality or facts, blocking external events from awareness. This mechanism is often used when the reality is too difficult to handle or the individual is not ready to accept it. For example, upon receiving bad news, like the serious injury of a loved one, a person might initially respond with "No, it can't be true," as a way to temporarily shelter themselves from the emotional impact of the situation.