What type of error does the phrase "I do not understand this word" represent?

The phrase "I do not understand this word" is a complete thought that contains a subject ("I") and a predicate ("do not understand this word"). It stands on its own as a fully formed sentence that expresses a clear idea. A complete sentence is characterized by having both a subject and a verb, and it conveys a complete thought without needing additional words to clarify its meaning. The phrase meets all these criteria, confirming that it is indeed a complete sentence. This clarity and grammatical correctness is what makes it the right answer.