Final answer:
The one word that fits all three sentences is "ruled," functioning as a verb to indicate marking with a line, making a decision in court, and governing a country.
Step-by-step explanation:
The one word that can be appropriately used in all three sentences provided is "ruled.” Here is how it would fit in each sentence:
- Tom ruled a line under his work to show that it was completed.
- The judge ruled that the boy was too young to appear in court as a witness.
- The king ruled his country for over fifty years and was much loved by his subjects.
The word "ruled" acts as a verb in these contexts, indicating that Tom made a clear line, the judge made a formal decision, and the king governed his country.