Final answer:
The most powerful theme in 'The Gift of the Magi' is 'Unselfish love is the greatest of all gifts', illustrated by the selfless sacrifices made by both main characters.
Step-by-step explanation:
The story The Gift of the Magi by O. Henry presents several themes, but the most evident and powerful theme is encapsulated in option D: Unselfish love is the greatest of all gifts. This theme is characterized by the selfless acts of both main characters, Jim and Della, who each sacrifice their most prized possessions in order to buy a Christmas gift for the other. Della sells her beautiful hair to buy a platinum fob chain for Jim's watch, and Jim sells his watch to buy combs for Della's hair. This ironic twist highlights the depth of their love and the idea that the value of a gift lies in the sacrifice and thought behind it, rather than its material worth.