To respond to this statement, the user needs to provide an appropriate phrase that conveys their agreement or disagreement with the statement. The phrase should also be grammatically correct and make sense in the context.

Now, let's go through each option and explain why it is right or wrong:

A. I don't think it. This option is grammatically correct, but it does not provide a clear response to the statement. It is not clear what "it" refers to, and the phrase does not convey whether the user agrees or disagrees with the statement.

B. I don't think. This option is grammatically correct, but it also does not provide a clear response to the statement. The phrase is incomplete and does not convey whether the user agrees or disagrees with the statement.

C. I don't think so. This option is the correct answer. It is grammatically correct and provides a clear response to the statement. The phrase "I don't think so" conveys the user's disagreement with the statement that most restaurants are too expensive.

D. I don't think as it. This option is grammatically incorrect. The phrase "I don't think as it" does not make sense in the context and does not provide a clear response to the statement.

Therefore, the answer is: C. I don't think so.