Final answer:
The key that unlocks the cupboard above was given to the boy by his mother.
Step-by-step explanation:
The key that unlocks the cupboard above was given to the boy by his mother. The passage describes how the boy's mother gave him the key and where it was kept. She said, "the key is down by the front steps, under a plantain leaf." This information indicates that the key was given to the boy before his mother actually physically handed it to him and that it was kept in a specific location before being given to him.