Write else’s. Use of apostrophe is necessary here.

The sentence has an error in the fourth part which is grammatically incorrect. The correct version of the sentence should be “He acted not as per my advice, but as per somebody else”.

The sentence has four parts and the error is in the fourth part. The error is grammatical in nature. The structure of the sentence should be “He acted not as per my advice, but as per somebody else”. The conjunction ‘but’ is used to join two contrasting ideas. The idea in each part should be contrasting to each other. The first part of the sentence is “He acted not” which is in negative form. The second part of the sentence is “as per my advice”, which tells us that the person did not act as per the advice given by the speaker. The third part of the sentence is “but” which is used to join two contrasting ideas. But the fourth part of the sentence is “somebody else”, which is grammatically incorrect because it does not give a contrast to the first part of the sentence, which is in negative form. The correct version of the sentence should be “He acted not as per my advice, but as per somebody else”.

Hence, the correct option is option D : 4.

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