The instrument that was the predecessor of the koto originated in China. In which country did the koto actually become a known and developed instrument?

Japan

Explanation:

Koto is traditional Japanese instrument.

It originated from a Chinese instrument called guzheng or zheng, and they remained very similar. They both have one large body and mechanism for tightening and loosening strings that produce a clear sound. The main difference is in the number of strings (guzheng has almost 10 more).

Koto became famous in the 8th century in Japan. It was played at the imperial court. Yet, in the 17th century, blind player Yatsuhashi Kengyo learned how to play it and compose on it, and the songs he made became Japanese traditional songs.

Korean musical instrument Gayageum is very similar and also related to the Japanese and Chinese versions.