Which of the following terms is best described as something that is part of a whole?

The term that best describes something that is part of a whole is "element." In various contexts, an element refers to a fundamental or essential part of a larger structure or system. For example, in chemistry, an element is a basic substance that cannot be broken down into simpler substances and is a building block of matter. In broader contexts, elements can pertain to components of a system, such as elements of art or literature, where each element plays a pivotal role in creating the entirety.

In contrast, a "subpart" typically indicates a division of something larger, which may not capture the essence of being a fundamental component. "Fragment" suggests a piece that may not necessarily contribute to the whole in a meaningful way, often implying that it is incomplete. "Substance," while it can denote a material or matter, doesn’t specifically convey the idea of being an integral component within a whole as effectively as "element."