John is persistently and excessively afraid of snakes. Whenever he sees a snake, he feels intense anxiety and thus avoids them at all costs. John realizes, however, that this fear is unrealistic. John most likely has

a specific phobia

Explanation:

Phobia is defined as a situation in which an individual experiences an irrational or intense or extreme fear related to particular situations or events. It is often distinguished as one of the different types of anxiety disorders encompassing anxiety as the main symptom. Phobia is generally considered as "learned emotional response".

Specific phobia is one of the distinct types of phobia in which a person feels fear related to a specific thing, event or situation. it can be caused by different reasons including genetic factors, environmental factors, etc. An individual might avoid a situation that creates phobia.

As per the question, John most likely has a specific phobia.