T.R. Malthus wrote
(A) An essay on population
(B) Origin of species
(C) Discovery of India
(D) Textbook of mathematics

Hint: This book was the first which was published in 1798. The debate on the size of a community was fuelled by the publication of this book. Later it helps in passing an act in Britain that is known as the Census Act which was activated in the year 1800.

Complete answer:
Let’s discuss the option and find the correct answer.
- An essay on population: The difficulties that can arrive in the future by increasing the size of the population or by increasing the geometric progression are mentioned. Besides, the increasing production of food results in arithmetic progression and thus there seems to be a difference between famine and fame (unless a decrease occurs in birth rate). It was the first book that was published on the population.
- Origin of species: In evolutionary biology, this work in scientific literature was published on 24th November 1859 by the scientist Charles Darwin. It has described that several natural selections are responsible for the change and evolution in the population.
- Discovery of India: The discovery of India was the book which is believed to contain the heritage and culture of India. It was written by Pandit Jawaharlal Nehru when he was imprisoned in jail during the year 1942 to 1945.
Hence Option (A) An essay on population is the correct answer.

Note: The full name of T.R.Malthus was Thomas Robert Malthus. He was an English scholar, cleric, and economist and contributed a lot in the field of population ecology.