The correct answer is C. Bargain.
To haggle is to negotiate over the price of something. It is a form of bargaining, which is the act of discussing something in order to reach an agreement. When you haggle, you are trying to get a better price for something by offering to pay less than the asking price. The other party may also haggle, and the two of you may go back and forth until you reach an agreement.
A. Postpone means to delay something until a later time.
B. Accept means to agree to something.
D. Reject means to refuse to accept something.
In the context of the question, the only option that makes sense is C. Bargain. The other options do not relate to the act of haggling.