学校は九時___始まります。 (School starts at nine o'clock.)

Answer and explanation

Correct answer: D. に

に marks a specific point in time such as 九時.
Answer: に.
Why not the others: で: で marks a place of action. を: を marks an object. が: が would mark a subject.

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学校は九時 始まります。 (School… | JLPT Practice (N5 and N4) - Japan