Systematic desensitization is a psychological technique used to treat phobias by gradually exposing the individual to the feared object or situation in a controlled and systematic manner. The goal is to help the person become less sensitive to the fear over time. This technique is based on the principle of classical conditioning.
Step 1: Analyze the options.
(A) Rationalization: This is a defense mechanism where a person justifies their behaviors or feelings with logical explanations, but it is not a treatment for phobias.
(B) Sublimation: This is another defense mechanism where socially unacceptable impulses are transformed into socially acceptable behaviors, but it is not specifically used to treat phobias.
(C) Systematic desensitization: This is the correct technique used to treat phobias, involving gradual exposure to the fear.
(D) Improve thinking: This is a vague term and is not a specific psychological technique used to treat phobias.
Step 2: Conclusion.
Systematic desensitization is the correct technique for treating phobias, as it involves gradual and controlled exposure to the feared stimulus.
Final Answer:
\[
\boxed{(C) \, \text{Systematic desensitization}}
\]