Concept:
The word reign refers to the period during which a king, queen, emperor, or ruler exercises authority over a kingdom or empire. It emphasizes the act of ruling rather than merely the passage of time.
Step 1: Understand the underlined word.
The sentence says:
\[
\text{"It happened in the reign of Emperor Ashoka."}
\]
This indicates that the event occurred while Emperor Ashoka was governing his empire.
Step 2: Analyze each option.
• Rule -- exercise of authority by a ruler.
• Period -- only indicates time, not governance.
• Kingdom -- the territory ruled.
• Family -- completely unrelated.
Step 3: Select the best meaning.
Since reign specifically refers to the act and duration of ruling, the closest meaning is rule.
\[
\boxed{\text{Rule}}
\]