Final answer:
The phrase 'so easy to read' signifies that the mother thought the book was easy to comprehend, suggesting a smooth and understandable reading experience.
Step-by-step explanation:
The phrase 'so easy to read' used by the author's mother likely conveys that the book was easy to comprehend. This type of comment typically implies that the text was clear, straightforward, and understandable, without complex language or themes that might make it difficult to follow. In the multiple-choice options provided with the question, the correct answer would be (d) The book was easy to comprehend. It suggests that the mother found the reading experience smooth and possibly enjoyable because of the book's accessibility.