Once the ion exchange resin can no longer remove hardness, it is said to be:

• A. Activated
• B. Exhausted
• C. Depleted
• D. Full

Answer: (B)

When an ion exchange resin has reached its capacity to remove hardness from water, it is referred to as "exhausted." This exhaustion occurs after the resin has exchanged all of its available functional sites with hardness ions (such as calcium and magnesium), meaning it can no longer effectively remove these ions from the water supply. The resin is essentially at the end of its operational life in terms of taking hardness out of the water, necessitating a regeneration process to restore its ion exchange capabilities.

The other terms do not accurately describe the state of the resin once it can no longer perform its function. "Activated" refers to resin that is ready to exchange ions, while "full" may imply a different context that does not clearly convey the inability of the resin to function. "Depleted" can also imply a reduction in function but is less commonly used specifically in the context of ion exchange processes. Thus, "exhausted" is the most appropriate term to indicate that the resin can no longer perform its intended function of hardness removal.