Step 1: Understanding the book
Gulamgiri is a famous work by Jyotirao Phule, an Indian social reformer. The book addresses the issue of untouchability and the social discrimination faced by the lower castes in India.
Step 2: Context of the book
In Gulamgiri, Phule talks about the oppressive social structures in Indian society and advocates for the abolition of untouchability. His work was part of a broader movement for social reform and the upliftment of the marginalized classes.
Step 3: Other options explanation
- (A) Caste System: While the book critiques the caste system, it specifically focuses on untouchability.
- (B) Communalism: The book does not address communalism but focuses on social discrimination and untouchability.
- (D) Child Marriage: Although Phule worked on issues related to social reform, Gulamgiri is not about child marriage.
Step 4: Conclusion
Thus, the correct answer is:
\[
\boxed{\text{Untouchability}}
\]