Which of the following describes an atom?

An atom is defined as the smallest part of an element that retains all the properties of that element. It consists of a nucleus, containing protons and neutrons, surrounded by electrons. This understanding of an atom is foundational in chemistry, as each element on the periodic table is comprised of atoms unique to that element, providing its distinct characteristics.

In contrast, the other options describe different concepts: the first option refers to a compound, which is formed when two or more different elements bond together, indicating that a compound consists of multiple atoms. The third option describes a group of molecules, which are formed from two or more atoms bonded together, highlighting the complexity beyond a singular atom. The last option pertains to biological organization, where tissues are complexes made up of cells, which are in turn made up of atoms.

Thus, the definition that an atom is the smallest part of an element aligns perfectly with its scientific characterization, making this option the correct answer.