Final answer:
The most effective buffer at resisting pH changes when acid or base is added will be one with equal concentrations of a weak acid and its conjugate base. In this case, the mixture of 0.1M HA and 0.1M A is the best buffer, as it maintains near equal concentrations after acid addition and has a higher overall buffer concentration.
Step-by-step explanation:
When choosing which solution is a better buffer, it's important to consider the resistance to pH changes upon the addition of acid or base. According to buffer action principles, a solution that has about equal concentrations of a weak acid and its conjugate base, often in the form of its salt, is more effective. Upon adding 0.001 M HCl, response 3, with a mixture of 0.1M HA and 0.1M A, is the most resistant as it maintains near equal concentrations post-addition, changing from [A] to 0.099 M and [HA] to 0.101 M. Moreover, a high overall buffer concentration enhances the buffer capacity.