What happens when an acid and a base react together?
A salt and water are formed correctly describes the effect or change asked about in this question. In Acid, Base, pH, understanding cause-and-effect relationships is essential for predicting biological outcomes. A salt and water are formed occurs because of specific molecular interactions, thermodynamic principles, or regulatory mechanisms that govern this biological process. The other options (A buffer is formed, The solution becomes acidic, and The solution becomes basic) describe either opposite effects, effects that occur under different conditions, or changes associated with unrelated processes.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 2