Answer:
The author includes these sentences in the story mainly to show that Catherine thinks it is important to help others.
Step-by-step explanation:
The short story "The Mysterious Box" tells how the two siblings Samuel and Catherine discovered "a hidden treasure" in their new house. And though this treasure was different from what Samuel wanted and expected to find, they nevertheless helped recover a lost treasure of the previous owner.
In the given excerpt from the story, the siblings found a trunk while cleaning the attic. And upon discovering clothes still in good condition, "Catherine decided to donate them to the school's drama department." This shows how considerate Catherine is, and her belief in the importance of helping others.
Thus, the correct answer is option C.