Have the box broken.

Answer: Option C

The correct answer is C: Get someone to break the box.
Let's break down why:
The original sentence "Have the box broken" is in a causative form.
This means it describes someone causing something to be done, rather than doing it themselves.
Think of it as "Arrange for the box to be broken."
Option A: "Have the broken box" describes a box that is already broken.
It doesn't imply someone caused it to be broken.
Option B: "Break the box" is a direct command.
It doesn't show someone else causing the action.
Option D: "They have broken the box" is a simple active voice sentence.
It states who did the breaking directly, not an arrangement for someone else to do it.
Option C: "Get someone to break the box" perfectly conveys the causative meaning.
It means you are asking or instructing someone else to perform the action of breaking the box.
Therefore, option C is the best explanation of the original sentence.