Symbols are the notations used to depict any of the elements. For example, hydrogen atom is depicted by the symbol, H and nitrogen atom by the symbol, N, etc. This notation is used to write the chemical formula of the compounds. Therefore, option (a) is incorrect.
An element is a pure substance made up of only one type of atoms. Therefore, option (c) is incorrect.
Atoms of same element that contain different number of neutrons are called isotopes. Therefore, option (b) is incorrect.
A compound is formed when a substance contains two or more different types of atoms combined in a fixed proportion. Here, physical and chemical properties of compounds are different form those of the elements from which they are formed. Therefore, option (d) is correct.
Hence, two or more different atoms are combined in definite proportion in any (d) compound.