Choose the most appropriate adverb to fill in the blank in the following sentence:-

He worked ______ for ten years on his research study.

relentlessly

The correct answer is Option (1) → relentlessly

• Relentlessly → tirelessly, with determination → fits perfectly with long, consistent hard work.

• In excitement → awkward phrase, not suitable.

• In the morning every day → indicates time, not the manner of working.

• To and fro → means back and forth, not suitable here.