What is the simplest type of substance that is represented by a specific letter or combination of letters?

The simplest type of substance that is represented by a specific letter or combination of letters is an element. Elements are pure substances that cannot be broken down into simpler substances by chemical means. Each element is defined by its unique atomic number, which corresponds to the number of protons in its nucleus. The letters that represent elements on the periodic table, known as chemical symbols, consist of one or two letters; for example, "H" stands for hydrogen, and "O" stands for oxygen.

In contrast, compounds are substances formed when two or more elements are chemically bonded together, requiring at least two different letter combinations to represent them, such as H2O for water. Mixtures comprise multiple substances physically combined, not represented by a single letter or combination, while ions are charged particles formed from atoms that have gained or lost electrons but are not the simplest substances, as they are derived from elements. Therefore, the defining characteristic of an element is its simplicity and representation by a specific symbol, affirming that it is the correct answer.