Complete the phrase: I can't believe I ate the ____ thing.

The phrase "I can't believe I ate the ____ thing" is best completed with "whole" because it conveys the idea that the speaker consumed the entirety of something, emphasizing the extent of their action. Using "whole" suggests a complete and undivided item, which fits naturally in the context of having eaten something entirely.

While "entire" also means complete, its use in this specific context feels slightly more formal and might not flow as seamlessly in colloquial speech. "All" does not work here because it does not fit grammatically in the phrase; the word "all" would typically precede a plural noun for it to make sense. Similarly, "every" generally requires a plural noun or a reference to items in a group rather than a singular noun like "thing." Hence, the most appropriate and natural word in this scenario is "whole."