Answer:
Zone.
Step-by-step explanation:
A buffer which is a blend of a weak acid and its conjugate base or vice versa is meant to resist pH changes i.e. H+ levels over a portion of pH scale when an acid or base is added to it. This level up to which a buffer can resist pH change or H+ change is know as buffering zone of that buffer.
For example:
Acetate buffer can resist a pH change between 3.76 to 5.76 on pH scale.