What is the correct spelling of the missing word? "His soothing words and manner were able to calm even the most ______ dog."

The correct spelling of the missing word is "vicious." This term is appropriately used in the context provided, as it describes a dog that is aggressive or ferocious. The spelling "vicious" consists of the root "vic-" which is common in words that imply cruelty or malice.

In contrast, the other options contain misspellings that detract from the standard English language usage: "vishus" lacks the appropriate representation of the "c" sound in the root, "vicous" incorrectly drops the second "i," and "viciouss" improperly adds an additional "s" at the end. The correct spelling, "vicious," effectively conveys the intended meaning of describing a dog that could be aggressive or threatening, which aligns with the context of the sentence about calming a difficult animal.