Explanation:

- He was keen in bringing about an end to the conflict: This suggests he was eager or enthusiastic, but doesn't fit grammatically with "keen."

- He was instrument in bringing about an end to the conflict: "Instrument" as a noun doesn't fit here; the adjective "instrumental" is needed.

- He was popular in bringing about an end to the conflict: This indicates he was well-liked, which doesn't relate directly to ending the conflict.

- He was instrumental in bringing about an end to the conflict: This means he played a crucial role, making it the correct fit.