In a nuclear reaction, the mass of the system before the reaction must be greater than or equal to the mass of the system after the reaction (mass-energy conservation). The mass of the original nucleus \( X \) is always greater than the sum of the masses of the products \( Y \) and \( Z \). This is because some mass is converted to energy during the reaction, which is released as energy (binding energy of the products).
Thus, the relation that holds true is:
\[
M_x<(M_y + M_z)
\]
Final Answer: (D) \( M_x<(M_y + M_z) \)