Answer:
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.
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