Hint: In 1798, this was the first book to be published by him, by the publication of this book there was fuel for the debate on the size of a community. In the year 1800, it helped in passing an act in the country Britain and the act is known as the census act.
Complete answer: Let us study the options to find the correct answer: Option A: This book is on facing the difficulties arrived by the increase in population size and geometric progression increment as well as the increase in arithmetic progression of food production, so this shows the difference between the fame and famine and this was the book which was written by T. R. Malthus on the population. Therefore, this is the correct answer.
Option B: As mathematics is a different subject and the author T.R. Malthus was not a mathematician so he has not written any mathematics book. Therefore, this is an incorrect answer.
Option C: Geography is a subject which is related to the discoveries of earth or discoveries of India, with this aspect he is not the writer of any geography book. Therefore, this is an incorrect answer.
Option D: Genetics is the aspect that is a branch of biology and related to the study of genes, genetic regulation, etc. therefore this is an incorrect answer.
Thus, the correct answer is option (A) Population.
Note: Thomas Robert Malthus was the full name of T.R. Malthus and he was a scholar in English, cleric, and economics as well as he had a great contribution in the field of population ecology. Malthus countered this belief arguing that segments of the general population have always been invariably poor and miserable, which effectively should cause population growth.