Step 1: Understanding the Word.
"Relegate" comes from Latin roots meaning to send away or send back, and in modern English it is used when something or someone is pushed down to a lower rank, position, or level of importance.
Step 2: Key Approach.
The best way to fix a word's meaning is to recall it in a familiar sentence. Sports commentary often says a team was "relegated" after a poor season, meaning the team moved down a division, that is, given a lower standing.
Step 3: Detailed Explanation.
Applying that same sense here, the Celts in the passage have been "relegated" in importance in the historical record, meaning their significance has been pushed down or downgraded compared to other periods or peoples. Checking the options, "to make importance" is grammatically odd and means close to the opposite of the word, "delegate" is a different word entirely about handing over a task, and "give poor treatment" describes mistreatment rather than a change in rank or standing. Only "to make less important" captures the core sense of lowering someone's or something's status.
Step 4: Final Answer.
"Relegate" means to make less important, so the answer is option (b).