The correct answer is A. He said that the mice will play when the cat is away.
The sentence “The mice will play, when the cat is away” is in the simple future tense. This means that the speaker is predicting that the mice will play at some point in the future, after the cat has left.
The other options are incorrect because they use different tenses. Option B uses the past tense, which would indicate that the speaker is talking about something that happened in the past. Option C uses the conditional tense, which would indicate that the speaker is talking about something that is possible but not certain. Option D uses the modal verb “shall,” which is used to express obligation or necessity.
In conclusion, the correct answer is A. He said that the mice will play when the cat is away.