Final answer:
O. Henry suggests Jim and Della are the wisest for their selfless acts of giving, sacrificing their most treasured possessions for each other, signifying deep love and selflessness. Option number b is correct.
Step-by-step explanation:
When O. Henry writes that Jim and Della are the wisest in their gift-giving, he is emphasizing the depth of their love and selflessness. They did not give the most expensive gifts, nor were they cunning in their strategy or popular in their community.
Instead, they were willing to sacrifice their most prized possessions in order to give something meaningful to the other, which is a true demonstration of selflessness. Therefore, the correct answer is B) Because they were the most selfless in their giving.