Dulong and Petit’s law is valid only for:
A. Metals
B. Non-metals
C. Gaseous elements
D. Solid elements

Hint: The Dulong and Petit’s law states that the molar specific heat capacity of any solid element is equal to the thrice of the ratio of universal gas constant and the molar mass. In short, the heat capacity per mole of various solid elements is equal to thrice of the gas constant or simply 3R.

Complete answer:
Dulong and Petit framed a graph between molar heat capacity along the y-axis and atomic number along the x-axis. They found that the molar heat capacity of various elements was nearby equal to a constant value and the value is equal to 3R. Mathematically, they stated that:
$\begin{gathered}
  M \approx 3R \\
  M \times c \approx 25J/g{}^oC \\
\end{gathered} $
Where, M = atomic mass of the element.
C = specific heat capacity of the element (J/ K)
R = universal gas constant (J/mol K)
After the evolution of Kinetic theory of gases, the values of gas constant were defined and then the statement of Dulong and Petit was modified further, thus, assigning some particular values for each element. These calculations were made by the respective scientists for the solid elements.

So, the correct answer is “Option D.

Note:
Thus, one must keep in mind that the Dulong and Petit’s law was framed to determine the atomic masses of the solid elements using their specific heat capacities and after the introduction of kinetic theory of gases, the values were numerically fixed.