When a person defends against anxiety by adopting behaviour opposite to one's true feelings it is known as :

Reaction formation

In reaction formation, a person defends against anxiety by adopting behaviours opposite to her/his true feelings. A person with strong sexual urges, who channels her/his energy into religious fervour, presents a classical example of reaction formation.

The other options are not as good because:

• Repression: Repression is the unconscious pushing away of unwanted or painful thoughts, memories, or emotions.
• Denial: Denial is the refusal to acknowledge or accept reality.
• Projection: Projection is the unconscious attributing of one's own unacceptable thoughts, feelings, or behaviors to others.