The correct answer is C) prevent threatening thoughts from attaining awareness.
If you ask Freud, the purpose of defense mechanisms is to prevent threatening thoughts from attaining awareness.
According to psychologists Anna Freud, the purpose of defense mechanisms is some type of auto defenses that protect an individual, unconsciously. The protection is against anxiety or not desired thoughts or feelings, even guilt. Among those mechanisms we find, displacement, denial, repression, regression, or projection.
Anna Freud is the daughter of the renowned psychologist, Sigmund Freud.