Final answer:
The best buffer solution is created by combining a weak acid and its conjugate base, or a weak base and its conjugate acid, because these mixtures are able to resist pH changes when strong acids or bases are added. Therefore, correct options are c or d.
Step-by-step explanation:
The best buffer solution when combined together would be c) Weak acid and its conjugate base or d) Weak base and its conjugate acid.
These combinations resist changes in pH when small amounts of strong acids or strong bases are added due to the Common Ion Effect, which is the principle that a buffer works by maintaining a stable concentration of hydronium ions.
Examples of buffer solutions include a mixture of acetic acid (CH3COOH) and sodium acetate (CH3COONa), or ammonia (NH3) and ammonium chloride (NH4Cl). The most effective buffers contain equal concentrations of an acid and its conjugate base.
Strong acids and bases, on the other hand, do not make effective buffers because they dissociate almost completely in water, leaving no room for buffer action. The conjugate base of a strong acid and the conjugate acid of a strong base are typically very weak and do not contribute to buffering capacity.