**Core Concept**: Vocabulary (prefixes and root words). **Answer Analysis**: - **A. Illiterate** is correct. The prefix 'il-' (a variant of 'in-') negates the root 'literate,' forming the meaning 'unable to read/write.' - **B. Immoral** refers to unethical behavior, unrelated to literacy. - **C. Immune** relates to biological resistance, not education. - **D. Impatient** describes a lack of patience. **Common Mistake**: Confusing similar prefixes (e.g., 'il-' vs. 'im-').