What is the correct word choice in the sentence: "Grace is feeling so badly"?

In the context of the sentence, "Grace is feeling so badly," the correct choice is "Feeling so bad." This is because "bad" is an adjective that describes the state of being, which is what is needed in this construction. When discussing feelings or general states of being, it’s appropriate to use adjectives like "bad" rather than adverbs, which would be the case with "badly." This is a common point of confusion in English but is important for proper grammatical structure.

"Feeling so badly" suggests a focus on the manner in which someone feels, typically referring to performing an action poorly, which is not the intent here. The focus is on Grace's state of feeling rather than the way she feels. The other choices present "terribly," "ill," and "poorly," which might generally seem appropriate in different contexts but don’t fit as accurately into this particular phrase structure. "Ill" and "poorly" may imply sickness, whereas "feeling so bad" is the concise and grammatically correct expression for expressing a negative emotional or mental state.