Introduction:
Evans, the protagonist of the story "Evans Tries an O-Level" by Colin Dexter, devises a plan to escape from prison. The question asks how his plan could become a success, and the correct answer is option 'A' - because of his wits. Let's explore why this is the correct answer in detail.

Explanation:
Evans' plan of escape could become a success due to several factors, primarily his wits. Let's examine each of these factors:

1. Wits:
Evans is portrayed as an intelligent and resourceful individual throughout the story. His wits enable him to come up with a well-thought-out plan to escape from prison. He carefully observes his surroundings, evaluates the strengths and weaknesses of the prison's security systems, and uses his knowledge to outsmart the authorities. His ability to think quickly on his feet allows him to adapt to unexpected situations, increasing the likelihood of his plan's success.

2. Cunning:
Cunning, though not explicitly mentioned in the given options, is closely related to Evans' wits. His plan involves tricking the prison staff by posing as a student taking the O-Level exam. He uses his cunning to create a believable disguise, memorize information about the exam, and imitate the behavior of a student. This cunningness helps him evade suspicion and gain the trust of the prison staff, ultimately aiding in his successful escape.

3. Keeping his hat on his head:
While keeping his hat on his head is not the sole reason for the success of Evans' plan, it does play a significant role. Evans strategically wears his hat throughout the escape process to conceal his true identity and avoid detection. It becomes a crucial element of his disguise and contributes to his ability to blend in with the other students taking the exam. By keeping his hat on, Evans effectively maintains his cover and increases the chances of his plan's success.

Conclusion:
In conclusion, Evans' plan of escape becomes a success primarily due to his wits. His intelligence, resourcefulness, and ability to think quickly enable him to outsmart the prison authorities and execute his plan flawlessly. While his cunning and the strategic use of his hat also contribute to his success, it is ultimately his wits that play the most significant role in ensuring the accomplishment of his escape.