Step 1: Understanding the sentence.
The given sentence contains a reported speech where the speaker is warning the listener. The phrase "he said to her in a calm voice" indicates that the speaker is issuing the warning calmly.
Step 2: Analyzing the options.
(A) He warned her to shoot if she didn't keep quiet calmly: This option is incorrect because it changes the meaning and doesn’t capture the calm warning tone correctly.
(B) He said calmly that I shall shoot you if you don't be quiet: This option incorrectly uses "I shall shoot you," while the original sentence uses "I shall shoot you," which is grammatically incorrect here.
(C) He warned her calmly that he would shoot her if she didn't keep quiet: This is correct. The phrase accurately captures the calm tone of the original sentence and conveys the threat appropriately.
(D) Calmly he warned her that be quiet or else he will have to shoot her: This option is incorrect as it doesn’t correctly convey the structure of the original sentence.
Step 3: Conclusion.
The correct answer is (C) He warned her calmly that he would shoot her if she didn't keep quiet, as it best matches the meaning and tone of the original sentence.