Which defense mechanism would best be described as "expressing the opposite of what one really feels"?

The defense mechanism that is described as "expressing the opposite of what one really feels" is reaction formation. This mechanism involves a person behaving in a way that is opposite to their true feelings or desires, often as a means to cope with anxiety or conflict about those feelings. For example, an individual who has feelings of anger towards someone may instead treat that person with excessive kindness and friendliness. This behavior serves both to hide the true feelings and to protect the individual from the discomfort of acknowledging those feelings.

The other options do not fit this description. Repression involves burying unwanted thoughts or feelings in the unconscious, projection involves attributing one's own unacceptable feelings to someone else, and rationalization entails creating logical reasons or excuses for behaviors or feelings that are actually driven by less acceptable motives. Each of these mechanisms operates differently from reaction formation, which specifically requires a display of emotion that is opposite to one's true feelings.