The correct answer is C. He warned her calmly that he would shoot her if she didn’t keep quiet.
The sentence “If you don’t keep quiet I shall shoot you”, he said to her in a calm voice” is a threat. The speaker is threatening to shoot the listener if they do not keep quiet. The speaker is doing this in a calm voice, which suggests that they are serious about their threat.
Option A is incorrect because it does not accurately reflect the tone of the sentence. The speaker is not “calmly warning” the listener, they are threatening them.
Option B is incorrect because it does not accurately reflect the content of the sentence. The speaker is not saying “I shall shoot you if you don’t be quiet”, they are saying “If you don’t keep quiet I shall shoot you”.
Option D is incorrect because it does not accurately reflect the structure of the sentence. The speaker is not “calmly warning” the listener, they are threatening them.