Which of the following is a uniform substance that cannot be further decomposed by ordinary chemical means?

The correct answer is an element because it is defined as a pure substance that cannot be broken down into simpler substances by ordinary chemical reactions. Elements are the fundamental building blocks of matter and are represented on the periodic table. Each element has unique properties and consists of only one type of atom, making them homogeneous and pure.

In contrast, compounds are substances formed from two or more elements that are chemically bonded together and can be decomposed into their constituent elements through chemical reactions. Mixtures consist of two or more substances (elements or compounds) that are physically combined but not chemically bonded, and they can be separated by physical means. Solutions, a type of mixture, involve a solute dissolved in a solvent, which can also be separated by physical processes.