A buffer is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. ... It is used to prevent any change in the pH of a solution, regardless of solute. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.

Buffer solution is the correct answer to keep the pH constant. A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added to it. It consists of a weak acid and its conjugate base or a weak base and its conjugate acid. The weak acid or weak base component of the buffer reacts with any added acid or base, preventing the pH from changing significantly.

Here's a detailed explanation of why buffer solutions maintain a constant pH:

1. Composition of Buffer Solution:
- A buffer solution is made up of a weak acid and its conjugate base or a weak base and its conjugate acid.
- The weak acid donates protons (H+) and the weak base accepts protons (H+).
- The conjugate base of the weak acid and the conjugate acid of the weak base act as reservoirs for protons, allowing the buffer to resist changes in pH.

2. Acid-Base Equilibrium:
- In a buffer solution, there is an equilibrium between the weak acid and its conjugate base or the weak base and its conjugate acid.
- The weak acid partially dissociates, releasing some H+ ions, while the conjugate base accepts these H+ ions.
- This equilibrium allows the buffer to resist changes in pH because the addition of an acid or base is counteracted by the corresponding reaction in the equilibrium.

3. Buffering Capacity:
- The ability of a buffer solution to maintain a constant pH is determined by its buffering capacity.
- Buffering capacity depends on the relative concentrations of the weak acid and its conjugate base or the weak base and its conjugate acid.
- Higher concentrations of the weak acid and its conjugate base result in a greater buffering capacity.

4. pH Regulation:
- When an acid is added to a buffer solution, the weak base component of the buffer reacts with the added acid, neutralizing it and preventing a significant change in pH.
- Similarly, when a base is added to a buffer solution, the weak acid component of the buffer reacts with the added base, neutralizing it and maintaining the pH.

5. Range of Buffering:
- Buffer solutions have a specific pH range where they are most effective.
- This range is determined by the pKa value of the weak acid or weak base component of the buffer.
- The pKa is the pH at which the weak acid or weak base is half-dissociated.
- Buffer solutions are most effective when the pH is within one unit of the pKa.

In summary, a buffer solution can maintain a constant pH by resisting changes when small amounts of acid or base are added. The weak acid and its conjugate base or the weak base and its conjugate acid components of the buffer react with the added acid or base, preventing a significant change in pH.