(C) the
"The" is a stop word — a common word with negligible meaning that is removed during text preprocessing, as it adds no significant value to text analysis.
Stop words are high-frequency words (like the, is, a, an, of) that carry little meaningful information and are removed during text normalisation. The source (Q.7, Test Yourself) defines stop words as "words with negligible value that are often removed during preprocessing." In the sentence, she and reads and book carry meaning; only the is a typical stop word.