Answer:
.An acid/base conjugate pair with a pKa>7
Step-by-step explanation:
A buffer is a solution of either a weak acid and its salt, or a weak base and its salt. The reagents used to make a buffer is selected having the pKa of the conjugate acid/base pair in mind.
The ability of a buffer solution to work or maintain a nearly constant pH when a small amount of acid or base is added to the solution is greatest at the pKa and diminishes as the pH of the solution goes above or below the pKa. This is the reason why the given answer was selected.
If the buffer should maintain the pH above 7, then the conjugate acid--base pKa should be greater than 7.