B. negociar
The correct translation of "to negotiate" in Spanish is "negociar". It directly translates to the act of negotiating, which involves discussions and agreements between parties.
A. regatear refers to haggling or bargaining, usually over prices, and is not a direct translation of "to negotiate."
C. pagar means "to pay" and is unrelated to negotiation.
D. ahorrar means "to save" and is also unrelated to negotiation.
Thus, "negociar" is the best choice for "to negotiate."