(a) Rare / valuable words
X represents rare / valuable words — words that occur the least in the corpus but add the most value to the text analysis.
The graph follows Zipf's Law distribution in NLP. Stop words (high occurrence, low value) appear at the top-left; frequent words are in the middle; and at the bottom-right (lowest occurrence), point X marks rare/valuable words — they appear least but carry the most meaning. The textbook (Test Yourself Q8) confirms: "They occur the least but add the most value to the corpus." Examiners expect students to read the graph direction correctly and link low occurrence with high value.