The error is in the part "as efficient as in the past."
In this case, we can say that option D is the correct answer.
We can arrive at this answer as follows:
- The word "efficient" is an adjective.
- In this case, this word can present a characteristic of something or someone.
- However, through the context of the sentence, we can see that an adjective is not an adequate word to express the real meaning of the sentence.
- The adjective would refer to telephones and the sentence needs something that refers to the verb "working" modified and making it more specific.
- In this case, it would be necessary an adverb, which could modify the verb "working" and make the sentence more expressive and correct.
- An adverb that would do this correctly would be the word "efficiently."
It is important to point out that an adverb is an invariable word, capable of determining the circumstances in which a verbal action took place.
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