valley, gorge
谷（たに）、渓谷（けいこく）、谷底（たにそこ）
valley, gorge, valley floor
On-reading (コク) is used in compounds; kun-reading (たに) is used standalone or with okurigana. Appears in many compounds — memorize the key ones rather than the kanji in isolation.
From the JLP Quiz question bank — the word in the form it is actually tested in.
【文章③】 （駅で） 外国人：すみません、この電車は渋谷に止まりますか。
Answer: 次の次の駅
