Select the word or phrase that makes this sentence grammatically correct: 'When you add sugar to it, your cup of coffee becomes not only sweet but also fattening.'

The word "to" is a preposition that does not fit the context of adding emphasis to the qualities of the coffee. Its presence in the sentence is grammatically correct, but it does not enhance the parallel structure needed for the phrase "not only... but also." Thus, it does not serve the intended function in this context.

The word "becomes" serves as the verb in the sentence, indicating the change in the coffee's characteristics. However, it does not contribute to the duality expressed in the phrase "not only... but also." The verb is necessary for the sentence structure but is not the key to correcting the grammatical form being questioned.

The phrase "cup of" refers to the object being described and does not affect the grammatical structure of the comparison being made. While it is necessary for identifying what is being discussed, it does not address the parallelism that is central to the sentence's meaning.