教室に学生が十___います。 (There are ten students in the classroom.)

Answer and explanation

Correct answer: D. 人

People are counted with 人 (with irregular 一人 and 二人).
Answer: 人.
Why not the others: 個: 個 counts small objects. 匹: 匹 counts small animals. 台: 台 counts machines and vehicles.

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