Which of the following pairs can create a buffer solution that works effectively at a physiological pH?

A buffer solution is designed to resist changes in pH upon the addition of small amounts of acid or base, and it typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. In this context, a pair that achieves this within the physiological pH range (approximately 7.4) is essential.

The combination of ammonia (NH3) and ammonium chloride (NH4Cl) is effective because NH3 is a weak base, and NH4Cl provides NH4+, which is the conjugate acid of the base. This pair can effectively neutralize added acids or bases, helping to maintain a stable pH. The pKa of NH4+ is around 9.25, which indicates that the NH3/NH4Cl pair is suitable for buffering around physiological pH values.

In contrast, other combinations listed do not fulfill the requirements for forming an effective buffer at physiological pH. For instance, hydrochloric acid (HCl) is a strong acid, which does not participate in buffering because it dissociates completely, and sodium chloride (NaCl) does not contribute any acidic or basic capability. Similarly, sulfuric acid (H2SO4) is a strong acid