Which of the following statements are not true regarding compounds ?

(A) Compounds show the properties of constituent substances.

(B) Only elements can react to form compounds.

(C) The composition of each compound is always fixed.

(D) Compounds can be separated by physical and chemical methods.

The correct answer is (A), (B), (D).

To determine which statements are not true regarding compounds, let us analyze each of the given statements step-by-step:

Statement (A): "Compounds show the properties of constituent substances."
This statement is incorrect (not true). A chemical compound is a new substance formed by the chemical combination of two or more elements in a fixed ratio. The properties of a compound are entirely different from the properties of its constituent elements. For example, water ($H_{2}O$) is a liquid used to extinguish fires, whereas hydrogen is a highly inflammable gas and oxygen supports combustion.

Statement (B): "Only elements can react to form compounds."
This statement is incorrect (not true). Compounds can be formed not only by the reaction of elements with elements, but also by the reaction of compounds with elements, or compounds with other compounds. For example, carbon monoxide ($CO$, a compound) reacts with oxygen ($O_2$, an element) to form carbon dioxide ($C{O}_2$, a compound).

Statement (C): "The composition of each compound is always fixed."
This statement is correct (true). According to the Law of Constant Proportion (or Definite Proportions), a chemical compound always contains its constituent elements combined together in a fixed ratio by mass, regardless of its source or method of preparation.

Statement (D): "Compounds can be separated by physical and chemical methods."
This statement is incorrect (not true). The constituent elements of a compound are bound together by chemical bonds and cannot be separated into simpler substances by ordinary physical methods (such as filtration, evaporation, or magnetic separation). They can only be separated using chemical or electrochemical methods (such as electrolysis).

Thus, statements (A), (B), and (D) are not true regarding compounds.