The law of multiple proportions is applicable for (A) two elements forming more than one compound (B) a compound involving at least three elements (C) one element forming more than one type of molecule (D) two elements forming one compound

The Law of Multiple Proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.

• A) This option perfectly describes the condition for the Law of Multiple Proportions. For example, carbon and oxygen can form carbon monoxide (CO) and carbon dioxide (CO$_2$). In these compounds, a fixed mass of carbon combines with different masses of oxygen, and these masses of oxygen are in a simple whole-number ratio (1:2).
• B) The law specifically applies to two elements, not necessarily three or more.
• C) This describes allotropy, where one element exists in different molecular forms (e.g., O$_2$ and O$_3$), which is not the subject of the Law of Multiple Proportions.
• D) If two elements form only one compound, there are no multiple proportions to compare. The law requires the formation of more than one compound.

Thus, the law is applicable when two elements form more than one compound.

The final answer is $\boxed{\text{A}}$