Question

When he reached the school the bell _____(rang, rings)

Answer 1

When he reached the school the bell $\bf{rang}$.

Since, we are talking about past sense , Rang would be used instead of Rings.

Rings is present continuous form of ring.

Answer 2

HEY MATE!

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Question- Fill in the blanks using the correct form of verbs: When he reached the school the bell _____(rang, rings)

the given sentence is in Past tense so we will use the past simple tense of ring which is rang.

Answer- When he reached the school the bell rang.

Hope it helps you