To prepare a buffer solution of a desired pH, the most efficient choice is a weak acid whose $pK_a$ is close to the required pH . The pH of an acidic buffer is calculated using the Henderson-Hasselbalch equation: $\text{pH} = pK_a + \log \frac{[\text{Salt}]}{[\text{Acid}]}$ . The buffer has maximum capacity to resist pH changes when the concentrations of the salt and acid are equal, which makes $\text{pH} = pK_a$ . Since the required pH is 3.58, acetoacetic acid with a $pK_a$ of 3.58 is the best choice.