The correct answer is A. He said that he must go next week.
The other options are incorrect because they do not accurately reflect the meaning of the original sentence.
Option B, “He said that he must go the following week,” is incorrect because it changes the order of the words in the original sentence. The original sentence says “I must go next week,” but Option B says “He said that he must go the following week.” This changes the meaning of the sentence because it implies that the speaker is talking about someone else who must go next week, rather than himself.
Option C, “He said that he would have to go the following week,” is incorrect because it changes the tense of the verb. The original sentence uses the present tense, but Option C uses the future tense. This changes the meaning of the sentence because it implies that the speaker is talking about something that will happen in the future, rather than something that is happening now.
Option D, “He said that he was to go the following week,” is incorrect because it changes the mood of the sentence. The original sentence is in the indicative mood, but Option D is in the subjunctive mood. This changes the meaning of the sentence because it implies that the speaker is talking about something that is possible or probable, rather than something that is certain.