hill, mound
丘（おか）、丘陵（きゅうりょう）、砂丘（さきゅう）
hill, hilly terrain, sand dune
On-reading (キュウ) is used in compounds; kun-reading (おか) 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②】 緑ヶ丘自治会より、秋の地域交流祭りについてお知らせします。
Answer: 今年で20回目を迎える
