Final answer:
The simplest, most fundamental unit of matter is the atom, made of subatomic particles and capable of forming molecules when combined with other atoms.
Step-by-step explanation:
The simplest, most fundamental unit of composition of matter is the atom. An atom is the smallest unit of an element that retains the unique properties of that element and can participate in a chemical change. Subatomic particles such as protons, electrons, and neutrons make up an atom, and when two or more atoms combine, they form a molecule. Molecules are the smallest particles into which any matter can be divided while still retaining its chemical properties, like the molecule of water which consists of two hydrogen atoms and one oxygen atom bonded together.