Which of the following is a solution that contains a weak acid and its conjugate base?

A solution that contains a weak acid and its conjugate base is known as a buffer solution. Buffers play a crucial role in maintaining a stable pH in a solution, even when small amounts of acids or bases are added. This stability is due to the weak acid being able to neutralize added bases, while the conjugate base can neutralize added acids.

For instance, if acetic acid (a weak acid) is mixed with its conjugate base, acetate, the resulting buffer solution helps maintain a pH around the pKa of acetic acid. This property is vital in many biological and chemical systems where pH balance must be preserved.

The other options do not fit the criteria of having both a weak acid and its conjugate base. A neutral solution lacks the ability to resist changes in pH since it doesn't contain components that can react with both acids and bases. An excessive solution typically refers to a solution with a high concentration of solute, which doesn't specifically indicate the presence of a weak acid and its conjugate base. A concentrated solution simply refers to the amount of solute relative to the solvent, again not specifically related to the buffering action of weak acids and their conjugate bases.