Weak Acid	Formula	\(K_a\) at \(25^\circ\text{C}\)
Acetic acid	\(\text{CH}_3\text{COOH}\)	\(1.8 \times 10^{-5}\)
Dihydrogen phosphate ion	\(\text{H}_2\text{PO}_4^-\)	\(6.2 \times 10^{-8}\)
Ammonium ion	\(\text{NH}_4^+\)	\(5.6 \times 10^{-10}\)
Hydrogen carbonate ion	\(\text{HCO}_3^-\)	\(4.7 \times 10^{-11}\)

A biochemist is preparing a buffer solution to maintain a stable \(\text{pH}\) of \(7.40\) for an in vitro enzyme assay. The table above lists the acid-dissociation constants (\(K_a\)) for four weak acids at \(25^\circ\text{C}\). Which of the following conjugate acid-base pairs is most suitable for preparing this buffer, and why?