The correct answer is A. coins.
Numismatics is the study of coins, currency, and medals. It is a branch of archaeology and history that deals with the study of coins, their production, distribution, and use. Numismatists study coins to learn about the history of the people who made them, the places where they were used, and the economic and political conditions of the time.
Option B, numbers, is incorrect because numismatics is not the study of numbers. Numbers are a branch of mathematics that deals with the study of quantities and their properties.
Option C, stamps, is incorrect because numismatics is not the study of stamps. Stamps are a type of collectible that is used to pay for postage. Philately is the study of stamps.
Option D, space, is incorrect because numismatics is not the study of space. Space is the boundless three-dimensional extent in which objects and events have relative position and direction.