When you add sugar to it, your cup of coffee becomes not only sweet, also fattening.

The phrase "your cup of coffee becomes not only sweet, also fattening" contains a structure that requires parallelism to maintain clarity and correct grammatical form. The word "becomes" is pivotal here as it is the verb that indicates the transformation of the coffee's attributes.

In this context, "not only" introduces a correlative conjunction that pairs with "but also," which is often implied but not explicitly stated. To correctly convey the idea that the coffee becomes both sweet and fattening, the structure should be: "not only sweet but also fattening." The verb "becomes" correctly links both qualities of the coffee, ensuring the sentence maintains logical flow and grammatical accuracy.

Thus, the use of "becomes" is essential for the sentence to communicate the intended meaning effectively, making it the correct choice in this context.