What is an atom?

An atom is defined as the smallest unit of an element that retains the chemical properties of that element. Each atom consists of a nucleus, which contains protons and neutrons, surrounded by electrons in orbitals. This structure allows atoms to participate in chemical reactions and bond with one another, defining the behavior of different elements in accordance with the periodic table.

The significance of option B lies in its emphasis on the atom being the fundamental building block of matter, which is essential for understanding chemistry and the physical sciences. It supports the notion that all matter is composed of atoms, each characterized by the number of protons in its nucleus, which determines the element to which it belongs.

In contrast, the other options either inaccurately describe atoms or represent concepts that are not fundamental to the definition of an atom. For example, the first option refers to units of measurement rather than the structure of matter, and the other options describe categories that include or rely on the concept of atoms but do not define what an atom is. This makes the correct answer clear in its alignment with the established scientific understanding of atomic structure.