Which of the following best describes an element?

The choice that states an element is "the purest part of a substance retaining its properties" accurately captures the essence of what an element is in the context of chemistry. An element is a fundamental substance that cannot be broken down into simpler substances through chemical reactions. Each element is composed of atoms that have the same number of protons, which define their chemical identity.

This definition reflects the purity of an element; it consists of a single type of atom and retains specific properties that are characteristic of that element, such as its atomic structure and behavior in chemical reactions. For example, oxygen is an element that, when in its elemental form (O2), exhibits unique properties like supporting combustion and being essential for respiration in living organisms.

In contrast, other options describe characteristics of compounds, mixtures, or imply the possibility of further breakdown, which do not align with the definition of an element. A complex substance that can be broken down refers more to compounds or mixtures, while dividing a substance into parts typically pertains to mixtures or compounds rather than pure elements. Thus, the best description of an element is one that emphasizes its purity and intrinsic properties, affirming why the selected answer is indeed the most accurate.