Explanation:

When two or more atoms of different elements combine with each other in a fixed ratio, a molecule of a compound is obtained. Let's break down the explanation into the following sections:

Atoms:
An atom is the smallest unit of an element that retains the chemical properties of that element. Each atom consists of a nucleus containing positively charged protons and uncharged neutrons, surrounded by negatively charged electrons. Atoms are represented by their atomic symbols, such as H for hydrogen, O for oxygen, and C for carbon.

Elements:
Elements are pure substances that cannot be broken down into simpler substances by chemical means. Each element is made up of only one type of atom. For example, oxygen (O) is an element composed of only oxygen atoms.

Compounds:
Compounds are substances formed when two or more different atoms chemically bond with each other in a fixed ratio. The atoms in a compound are held together by chemical bonds, which can be ionic or covalent. Compounds have unique properties that are different from those of the individual elements that make them up. For example, water (H2O) is a compound composed of two hydrogen atoms and one oxygen atom.

Molecules:
A molecule is the smallest unit of a compound that retains the chemical properties of that compound. Molecules can consist of two or more atoms of the same element (e.g., O2) or different elements (e.g., H2O). In the given question, when two or more atoms of different elements combine with each other in a fixed ratio, a molecule of a compound is obtained.

Therefore, the correct answer is option 'A' - Compound.