The book that the Professor slipped into his pocket before leaving the library is option 'A', Bakhar.

Explanation:
The question provides us with a list of options, and we are asked to identify the book that the Professor took with him. The correct answer is option 'A', Bakhar.

Bakhar is a Marathi historical narrative written by Mahipati in the 18th century. It is a historical account that describes the reign of Shivaji, the founder of the Maratha Empire in Western India. The book is highly regarded for its detailed and accurate portrayal of historical events and figures.

The Professor's choice of taking Bakhar with him is significant for several reasons:

1. Historical Interest:
As a professor, it is likely that he has a deep interest in history. Bakhar offers valuable insights into the life and times of Shivaji and the Maratha Empire. It provides a firsthand account of the events that shaped the region's history.

2. Research or Teaching:
The Professor may need Bakhar for his research or teaching purposes. Historical narratives like Bakhar are often used as primary sources to study and understand historical events. By having the book with him, he can refer to it whenever necessary and use it to enhance his knowledge or pass it on to his students.

3. Personal Interest:
It is possible that the Professor simply enjoys reading historical narratives like Bakhar. The book may have caught his attention during his time in the library, and he decided to take it home for personal reading and enjoyment.

In conclusion, the Professor slipped the book Bakhar into his pocket before leaving the library. His choice of taking this particular book suggests his interest in history, its potential relevance to his research or teaching, and possibly his personal enjoyment of historical narratives.