Which of the following statements is true?

A. A buffer is best used in pH ranges from two units below up to one unit above its pKa.
B. A buffer works best in a pH range farthest from its pKa.
C. A buffer is best used in pH ranges from one unit below up to one unit above its pKa.
D. A buffer works best in a pH range closest to its pKa.

Option B is correct.

A buffer is best used in pH ranges from one unit below up to one unit above its pKa.

Explanation:

The ability of a buffer solution to maintain a nearly constant pH when small amounts of acids or bases are added to the solution is greatest at the pKa & reduces as the pH of the solution goes above or below the pKa. A rule-of-thumb is to use a buffer within 1 pH unit of the pKa to maximize its buffering capacity.

Hope this Helps!