Answer = A
Explanation:
As buffers are solutions which resist the change in pH when a small amount of acid or base ( often strong) are added to it.
Reason for selecting option A is that, Buffer solution has a specific capacity to resist the change in pH both either acid or base is added to it and maintains its effectiveness. Its capacity is up to a particular level, when this level is crossed then buffer solution looses its capacity and becomes ineffective.
