Which of the following books is based on Astrology?

Astrology is a field that involves the study of celestial objects and their positions to predict and understand human affairs and natural phenomena. Several ancient Indian texts and books are based on astrology and provide insights into this field.

Among the options provided:

Option A: Romak Siddhanta
The Romak Siddhanta is an ancient treatise on astronomy, not specifically focused on astrology. It deals with astronomical calculations and theories.

Option B: Aryabhattiya
The Aryabhatiya, written by the Indian mathematician and astronomer Aryabhata, is a significant work in Indian astronomy. While it contains astronomical concepts, it is not primarily an astrology book.

Option C: Hora Shastra
The Hora Shastra, also known as Jyotisha Shastra, is a book dedicated to astrology. It deals with the study of horoscopes, planetary positions, and their influence on human lives and events.

Option D: Brahma Siddhanta
The Brahma Siddhanta is an ancient text that includes astronomical and mathematical concepts. While it contains astronomical calculations, it is not specifically focused on astrology.

Therefore, the correct answer is Option C: Hora Shastra, as it is a book based on astrology.